Develop Reference

Not able to initiate the List<customer>. The value of Customers.Names is null

using System;
using System.Collections.Generic;
using ConsoleApp3;
namespace ConsoleApp3
{
class Program
{
static void Main(string[] args)
{
Customers customers = new Customers();
List<Customers> names = new List<Customers>()
{
new Customers {Names = "Tanveer"},
new Customers {Names = "Nabila"},
new Customers {Names = "Suraj"}
};
foreach (int i = 0; i < names.Count; i++)
{
Console.WriteLine(customers.Names.Length);
}
}
class Customers
{
public string Names { get; set; }
}
}
}
I want to create the list of Customers and write it on the Console. But the Customers.Names is null. I am new to programming so please help.Thanks

First, let us fix up that naming. Naming is very important, but also one of the more annoying parts.
//Singular for the class
class Customer
{
//Also Singular, as this can only take 1 name
public string Name { get; set; }
}
//Plural, because it is a collection of Customer Instances.
List<Customer> Customers = new List<Customer>()
{
new Customer {Name = "Tanveer"},
new Customer {Name = "Nabila"},
new Customer {Name = "Suraj"}
};
Then you itterate over it using:
//Use a proper foreach, no need to deal with Indexes here
foreach (Customer current in Customers){
Console.WriteLine(current.Name);
}
If you do want to have a running counter, this is how the loop would look:
for(int i = 0; i < Customers.Count;i++){
//I you get any Exceptions, you want to split it up over 2 lines using a temporary variable
Console.WriteLine(Customers[i].Name);
}
What you had in your code was a bastardisation of for and foreach syntax, wich I doubt compiled.

Console.WriteLine(names[i].Names);

customers.Names is null because customers is an object which you haven't populated with any data, and has no obvious purpose. names is the actual list of customers with useful info in it.
names does not have a Names property directly either, though. The objects within the list do. So you need to refer to a specific object within the specific list.
And since you're in a loop for doing just that, names[i].Names.Length is no doubt what you intended.
N.B. However it needs to be a for rather than foreach in the loop definition - the syntax used with foreach is different. This shouldn't have compiled in order to allow you to even see the null output, so perhaps this is just a typo in your posted code.
for (int i = 0; i < names.Count; i++)
{
Console.WriteLine(names[i].Names.Length);
}
should be closer to what you need (although I'm not convinced you really are intending to print the length of each name...but that's up to you).
P.S. You should probably amend your naming convention so you've got Customer as the type and Name as the property. It's much more readable and comprehensible if they're singular rather than plural.

Sorting List Array based on an index of array

I want to sort a List Array on the basis of an array item.
I have a List Array of Strings as below:
List<String>[] MyProjects = new List<String>[20];
Through a loop, I have added five strings
(Id, Name, StartDate, EndDate, Status)
to each of the 20 projects from another detailed List source.
for(int i = 0; i<20; i++){
MyProjects[i].Add(DetailedProjectList.Id.ToString());
MyProjects[i].Add(DetailedProjectList.Name);
MyProjects[i].Add(DetailedProjectList.StartDate);
MyProjects[i].Add(DetailedProjectList.EndDate);
MyProjects[i].Add(DetailedProjectList.Status)}
The Status values are
"Slow", "Normal", "Fast", "Suspended" and "" for unknown status.
Based on Status, I want to sort MyProject List Array.
What I have done is that I have created another List as below
List<string> sortProjectsBy = new List<string>(){"Slow", "Normal", "Fast", "", "Suspended"};
I tried as below to sort, however unsuccessful.
MyProjects = MyProjects.OrderBy(x => sortProjectsBy.IndexOf(4));
Can anyone hint in the right direction. Thanks.

I suggest you to create class Project and then add all the fields inside it you need. It's much nicer and scalable in the future. Then create a List or an Array of projects and use the OrderBy() function to sort based on the field you want.
List<Project> projects = new List<>();
// Fill the list...
projects.OrderBy(project => project.Status);
The field Status has to be a primitive type or needs to implement the interface IComparable in order for the sorting to work. I suggest you add an enum for Status with int values.

First consider maybe to use Enum for status and put it in a different file lite (utils or something) - better to work like that.
enum Status {"Slow"=1, "Normal", "Fast", "", "Suspend"}
Now about the filtering you want to achieve do it like this (you need to tell which attribute of x you are referring to. In this case is status)
MyProjects = MyProjects.OrderBy(x => x.status == enum.Suspend);
Read about enums :
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/enum
Read about lambda expressions :
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions

First of all, storing project details as List is not adivisable. You need to create a Custom Class to represent them.
For example,
public class DetailedProjectList
{
public string Name {get;set;}
public eStatus Status {get;set;}
// rest of properties
}
Then You can use
var result = MyProjects.OrderBy(x=> sortProjectsBy.IndexOf(x.Status));
For example
List<string> sortProjectsBy = new List<string>(){"Slow", "Normal", "Fast", "", "Suspended"};
var MyProjects= new List<DetailedProjectList>{
new DetailedProjectList{Name="abc1", Status="Fast"},
new DetailedProjectList{Name="abc2", Status="Normal"},
new DetailedProjectList{Name="abc3", Status="Slow"},
};
var result = MyProjects.OrderBy(x=> sortProjectsBy.IndexOf(x.Status));
Output
abc3 Slow
abc2 Normal
abc1 Fast
A better approach thought would be to use Enum to represent Status.
public enum eStatus
{
Slow,
Normal,
Fast,
Unknown,
Suspended
}
Then your code can be simplified as
var MyProjects= new List<DetailedProjectList>{
new DetailedProjectList{Name="abc1", Status=eStatus.Fast},
new DetailedProjectList{Name="abc2", Status=eStatus.Normal},
new DetailedProjectList{Name="abc3", Status=eStatus.Slow},
};
var result = MyProjects.OrderBy(x=> x.Status);

Ok so you have a collection of 20 items. Based on them you need to create a list of strings(20 DetailedProjectList items).
What you can do to solve your problem is to SORT YOUR COLLECTION before you create your list of strings. In this way your list of strings will be sorted.
But your code is not optimal at all. So you should concider optimization on many levels.

Lets say you have ProjectDetail class as follow:
private class ProjectDetail
{
public int Id {get;set;}
public string Name {get;set;}
DateTime StartDate {get;set;} = DateTime.Now;
DateTime EndDate {get;set;} = DateTime.Now;
public string Status {get;set;}
public string toString => $"{Id} - {Name} - {StartDate} - {EndDate} - {Status}";
}
Notice that I have added a toString attribute to make things easier, and I also have added default values.
Then your program could be like:
static void Main(string[] args)
{
var projectDetails = MockProjectItems();
Console.WriteLine("Before sortig:");
foreach (var item in projectDetails)
{
Console.WriteLine(item.toString);
}
var myProjects = projectDetails.OrderBy(p => p.Status).Select(p => p.toString);
Console.WriteLine("\n\nAfter sorting:");
foreach (var item in myProjects)
{
Console.WriteLine(item);
}
}
where the helper method is
private static List<ProjectDetail> MockProjectItems()
{
var items = new List<ProjectDetail>(20);
for(int i = 0; i < 20 ; i += 4){
items.Add(new ProjectDetail{Id = i, Name = "RandomName "+i, Status = "Slow"});
items.Add(new ProjectDetail{Id = i+1, Name = "RandomName "+(i+1), Status = "Normal"});
items.Add(new ProjectDetail{Id = i+2, Name = "RandomName "+(i+2), Status = "Fast"});
items.Add(new ProjectDetail{Id = i+3, Name = "RandomName "+(i+3), Status = "Suspended"});
}
return items;
}
Then your program should print the following:

c# list in property which is array

This is my property class:
class Actions
{
public string[] Style { get; set; }
}
and this is my main method:
Actions action = new Actions();
List<string> list = new List<string>();
list.Add("one");
list.Add("two");
foreach (var item in list)
{
for (int i = 0; i < action.Style.Length; i++)
{
action.Style[i] = item.ToString();
Console.WriteLine(action.Style[i]);
}
}
How do I fill the property with list items?
This gives me a exception:
"object reference not set to an instance of an object".

There is no need to add your items one by one, you could just use the ToArray() method of your list like so:
List<string> list = new List<string>();
list.Add("one");
list.Add("two");
Actions action = new Actions {
Style = list.ToArray()
};

As has already been pointed out, Style is always null, given the code you have shared. #Eldeniz and #paul have shared different ways to fix that. Obviously, your sample code is just a sample fragment, so here are 2 other options you could consider if the previous two don't work for whatever reason (I'm just free-handing this, please excuse any typos).
1) You can have your Actions class always return a not-null object
class Actions
{
private string[] _style;
public string[] Style
{
get { return _style ?? new string[0]; }
set { _style = value; }
}
}
Note that this will allow you to always see the output of the style property as requested, assuming an empty array and null are, for your purposes, the same thing.
2) You can make your loop tolerant to null values
foreach (var item in list)
{
for (int i = 0; i < action?.Style.Length ?? 0; i++)
{
action.Style[i] = item.ToString();
Console.WriteLine(action.Style[i]);
}
}
Finally, just as a tip, if you have your debugger attached and you are stepping through your code, Visual Studio will help you pinpoint these sorts of errors pretty easily. Take the time to become friends with your debugger. If it gives you an error you don't understand, do a quick web search. Your future self will thank you.

You must create an instance of the Style property
List<string> list = new List<string>();
list.Add("one");
list.Add("two");
Actions action = new Actions();
action.Style=new string[list.Count];
foreach (var item in list)
{
for (int i = 0; i < action.Style.Length; i++)
{
action.Style[i] = item.ToString();
Console.WriteLine(action.Style[i]);
}
}

C# Removing a List from a List

Is there a convenient way to remove a nested list from another list if it meets certain requirements? For example, say we have a collection of stops, and we decide to call each collection of stops a route. Each route is in list from. Then we decide to put each route into a list as well.
So now that we have a list of routes, someone decides that certain types of routes really shouldn't be included in the route list. How can I remove those routes? Here's some sample code:
Example Class
public class Stops
{
public Stops(int _param1, string _param2)
{
param1 = _param1;
param2 = _param2;
}
public int param1 { get; set; }
public string param2 { get; set; }
}
Create the Lists
List<List<Stops>> lstRoutes = new List<List<Stops>>();
List<Stops> lstStops = new List<Stops>();
List<Stops> lstMoreStops = new List<Stops>();
// Create some stops
for (int i = 0; i < 5; i++)
{
lstStops.Add(new Stops(i, "some text"));
}
lstRoutes.Add(lstStops);
// Create some more stops
for (int i = 5; i < 10; i++)
{
lstMoreStops.Add(new Stops(i, "some more text"));
}
lstRoutes.Add(lstMoreStops);
How can I remove any route from lstRoutes that has, say, any param1 value greater than 6?

The simplest way (which can be applicable to all enumerables, not just lists) would be:
lstRoutes = lstRoutes.Where(r => !r.Any(s => s.param1 > 6)).ToList();
The snippet above creates a new list, so copying will occur which means both the performance and memory usage will slightly suffer. The most efficient way would be not adding those items to the list in the first place.
The second most efficient way would be to remove items from the list instead of constructing a new one, so the memory usage wouldn't be affected as much:
lstRoutes.RemoveAll(r => r.Any(s => s.param1 > 6));

List<Stops> stop = lstRoutes.Find(delegate(List<Stops> stp) { return stp.param1 > 6; });

Check for column name in a SqlDataReader object

How do I check to see if a column exists in a SqlDataReader object? In my data access layer, I have create a method that builds the same object for multiple stored procedures calls. One of the stored procedures has an additional column that is not used by the other stored procedures. I want to modified the method to accommodate for every scenario.
My application is written in C#.

public static class DataRecordExtensions
{
public static bool HasColumn(this IDataRecord dr, string columnName)
{
for (int i=0; i < dr.FieldCount; i++)
{
if (dr.GetName(i).Equals(columnName, StringComparison.InvariantCultureIgnoreCase))
return true;
}
return false;
}
}
Using Exceptions for control logic like in some other answers is considered bad practice and has performance costs. It also sends false positives to the profiler of # exceptions thrown and god help anyone setting their debugger to break on exceptions thrown.
GetSchemaTable() is also another suggestion in many answers. This would not be a preffered way of checking for a field's existance as it is not implemented in all versions (it's abstract and throws NotSupportedException in some versions of dotnetcore). GetSchemaTable is also overkill performance wise as it's a pretty heavy duty function if you check out the source.
Looping through the fields can have a small performance hit if you use it a lot and you may want to consider caching the results.

The correct code is:
public static bool HasColumn(DbDataReader Reader, string ColumnName) {
foreach (DataRow row in Reader.GetSchemaTable().Rows) {
if (row["ColumnName"].ToString() == ColumnName)
return true;
} //Still here? Column not found.
return false;
}

In one line, use this after your DataReader retrieval:
var fieldNames = Enumerable.Range(0, dr.FieldCount).Select(i => dr.GetName(i)).ToArray();
Then,
if (fieldNames.Contains("myField"))
{
var myFieldValue = dr["myField"];
...
Edit
Much more efficient one-liner that does not requires to load the schema:
var exists = Enumerable.Range(0, dr.FieldCount).Any(i => string.Equals(dr.GetName(i), fieldName, StringComparison.OrdinalIgnoreCase));

I think your best bet is to call GetOrdinal("columnName") on your DataReader up front, and catch an IndexOutOfRangeException in case the column isn't present.
In fact, let's make an extension method:
public static bool HasColumn(this IDataRecord r, string columnName)
{
try
{
return r.GetOrdinal(columnName) >= 0;
}
catch (IndexOutOfRangeException)
{
return false;
}
}
Edit
Ok, this post is starting to garner a few down-votes lately, and I can't delete it because it's the accepted answer, so I'm going to update it and (I hope) try to justify the use of exception handling as control flow.
The other way of achieving this, as posted by Chad Grant, is to loop through each field in the DataReader and do a case-insensitive comparison for the field name you're looking for. This will work really well, and truthfully will probably perform better than my method above. Certainly I would never use the method above inside a loop where performace was an issue.
I can think of one situation in which the try/GetOrdinal/catch method will work where the loop doesn't. It is, however, a completely hypothetical situation right now so it's a very flimsy justification. Regardless, bear with me and see what you think.
Imagine a database that allowed you to "alias" columns within a table. Imagine that I could define a table with a column called "EmployeeName" but also give it an alias of "EmpName", and doing a select for either name would return the data in that column. With me so far?
Now imagine that there's an ADO.NET provider for that database, and they've coded up an IDataReader implementation for it which takes column aliases into account.
Now, dr.GetName(i) (as used in Chad's answer) can only return a single string, so it has to return only one of the "aliases" on a column. However, GetOrdinal("EmpName") could use the internal implementation of this provider's fields to check each column's alias for the name you're looking for.
In this hypothetical "aliased columns" situation, the try/GetOrdinal/catch method would be the only way to be sure that you're checking for every variation of a column's name in the resultset.
Flimsy? Sure. But worth a thought. Honestly I'd much rather an "official" HasColumn method on IDataRecord.

Here is a working sample for Jasmin's idea:
var cols = r.GetSchemaTable().Rows.Cast<DataRow>().Select
(row => row["ColumnName"] as string).ToList();
if (cols.Contains("the column name"))
{
}

The following is simple and worked for me:
bool hasMyColumn = (reader.GetSchemaTable().Select("ColumnName = 'MyColumnName'").Count() == 1);

This works for me:
bool hasColumnName = reader.GetSchemaTable().AsEnumerable().Any(c => c["ColumnName"] == "YOUR_COLUMN_NAME");

I wrote this for Visual Basic users:
Protected Function HasColumnAndValue(ByRef reader As IDataReader, ByVal columnName As String) As Boolean
For i As Integer = 0 To reader.FieldCount - 1
If reader.GetName(i).Equals(columnName) Then
Return Not IsDBNull(reader(columnName))
End If
Next
Return False
End Function
I think this is more powerful and the usage is:
If HasColumnAndValue(reader, "ID_USER") Then
Me.UserID = reader.GetDecimal(reader.GetOrdinal("ID_USER")).ToString()
End If

If you read the question, Michael asked about DataReader, not DataRecord folks. Get your objects right.
Using a r.GetSchemaTable().Columns.Contains(field) on a DataRecord does work, but it returns BS columns (see screenshot below.)
To see if a data column exists AND contains data in a DataReader, use the following extensions:
public static class DataReaderExtensions
{
/// <summary>
/// Checks if a column's value is DBNull
/// </summary>
/// <param name="dataReader">The data reader</param>
/// <param name="columnName">The column name</param>
/// <returns>A bool indicating if the column's value is DBNull</returns>
public static bool IsDBNull(this IDataReader dataReader, string columnName)
{
return dataReader[columnName] == DBNull.Value;
}
/// <summary>
/// Checks if a column exists in a data reader
/// </summary>
/// <param name="dataReader">The data reader</param>
/// <param name="columnName">The column name</param>
/// <returns>A bool indicating the column exists</returns>
public static bool ContainsColumn(this IDataReader dataReader, string columnName)
{
/// See: http://stackoverflow.com/questions/373230/check-for-column-name-in-a-sqldatareader-object/7248381#7248381
try
{
return dataReader.GetOrdinal(columnName) >= 0;
}
catch (IndexOutOfRangeException)
{
return false;
}
}
}
Usage:
public static bool CanCreate(SqlDataReader dataReader)
{
return dataReader.ContainsColumn("RoleTemplateId")
&& !dataReader.IsDBNull("RoleTemplateId");
}
Calling r.GetSchemaTable().Columns on a DataReader returns BS columns:

TLDR:
There are lots of answers with claims about performance and bad practice, so I clarify that here.
The exception route is faster for higher numbers of returned columns, the loop route is faster for lower number of columns, and the crossover point is around 11 columns. Scroll to the bottom to see a graph and test code.
Full answer:
The code for some of the top answers work, but there is an underlying debate here for the "better" answer based on the acceptance of exception handling in logic and its related performance.
To clear that away, I do not believe there is much guidance regarding catching exceptions. Microsoft does have some guidance regarding throwing exceptions. There they do state:
Do not use exceptions for the normal flow of control, if possible.
The first note is the leniency of "if possible". More importantly, the description gives this context:
framework designers should design APIs so users can write code that does not throw exceptions
That means, if you are writing an API, that might be consumed by somebody else, give them the ability to navigate an exception without a try/catch. For example, provide a TryParse with your exception-throwing Parse method. Nowhere does this say though that you shouldn't catch an exception.
Further, as another user points out, catches have always allowed filtering by type and somewhat recently allow further filtering via the when clause. This seems like a waste of language features if we're not supposed to be using them.
It can be said that there is some cost for a thrown exception, and that cost may impact performance in a heavy loop. However, it can also be said that the cost of an exception is going to be negligible in a "connected application". Actual cost was investigated over a decade ago: How expensive are exceptions in C#?
In other words, the cost of a connection and query of a database is likely to dwarf that of a thrown exception.
All that aside, I wanted to determine which method truly is faster. As expected there is no concrete answer.
Any code that loops over the columns becomes slower as the number of columns increase. It can also be said that any code that relies on exceptions will slow depending on the rate in which the query fails to be found.
Taking the answers of both Chad Grant and Matt Hamilton, I ran both methods with up to 20 columns and up to a 50% error rate (the OP indicated he was using this two test between different stored procedures, so I assumed as few as two).
Here are the results, plotted with LINQPad:
The zigzags here are fault rates (column not found) within each column count.
Over narrower result sets, looping is a good choice. However, the GetOrdinal/Exception method is not nearly as sensitive to number of columns and begins to outperform the looping method right around 11 columns.
That said, I don't really have a preference performance wise as 11 columns sounds reasonable as an average number of columns returned over an entire application. In either case we're talking about fractions of a millisecond here.
However, from a code simplicity aspect, and alias support, I'd probably go with the GetOrdinal route.
Here is the test in LINQPad form. Feel free to repost with your own method:
void Main()
{
var loopResults = new List<Results>();
var exceptionResults = new List<Results>();
var totalRuns = 10000;
for (var colCount = 1; colCount < 20; colCount++)
{
using (var conn = new SqlConnection(#"Data Source=(localdb)\MSSQLLocalDb;Initial Catalog=master;Integrated Security=True;"))
{
conn.Open();
//create a dummy table where we can control the total columns
var columns = String.Join(",",
(new int[colCount]).Select((item, i) => $"'{i}' as col{i}")
);
var sql = $"select {columns} into #dummyTable";
var cmd = new SqlCommand(sql,conn);
cmd.ExecuteNonQuery();
var cmd2 = new SqlCommand("select * from #dummyTable", conn);
var reader = cmd2.ExecuteReader();
reader.Read();
Func<Func<IDataRecord, String, Boolean>, List<Results>> test = funcToTest =>
{
var results = new List<Results>();
Random r = new Random();
for (var faultRate = 0.1; faultRate <= 0.5; faultRate += 0.1)
{
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
var faultCount=0;
for (var testRun = 0; testRun < totalRuns; testRun++)
{
if (r.NextDouble() <= faultRate)
{
faultCount++;
if(funcToTest(reader, "colDNE"))
throw new ApplicationException("Should have thrown false");
}
else
{
for (var col = 0; col < colCount; col++)
{
if(!funcToTest(reader, $"col{col}"))
throw new ApplicationException("Should have thrown true");
}
}
}
stopwatch.Stop();
results.Add(new UserQuery.Results{
ColumnCount = colCount,
TargetNotFoundRate = faultRate,
NotFoundRate = faultCount * 1.0f / totalRuns,
TotalTime=stopwatch.Elapsed
});
}
return results;
};
loopResults.AddRange(test(HasColumnLoop));
exceptionResults.AddRange(test(HasColumnException));
}
}
"Loop".Dump();
loopResults.Dump();
"Exception".Dump();
exceptionResults.Dump();
var combinedResults = loopResults.Join(exceptionResults,l => l.ResultKey, e=> e.ResultKey, (l, e) => new{ResultKey = l.ResultKey, LoopResult=l.TotalTime, ExceptionResult=e.TotalTime});
combinedResults.Dump();
combinedResults
.Chart(r => r.ResultKey, r => r.LoopResult.Milliseconds * 1.0 / totalRuns, LINQPad.Util.SeriesType.Line)
.AddYSeries(r => r.ExceptionResult.Milliseconds * 1.0 / totalRuns, LINQPad.Util.SeriesType.Line)
.Dump();
}
public static bool HasColumnLoop(IDataRecord dr, string columnName)
{
for (int i = 0; i < dr.FieldCount; i++)
{
if (dr.GetName(i).Equals(columnName, StringComparison.InvariantCultureIgnoreCase))
return true;
}
return false;
}
public static bool HasColumnException(IDataRecord r, string columnName)
{
try
{
return r.GetOrdinal(columnName) >= 0;
}
catch (IndexOutOfRangeException)
{
return false;
}
}
public class Results
{
public double NotFoundRate { get; set; }
public double TargetNotFoundRate { get; set; }
public int ColumnCount { get; set; }
public double ResultKey {get => ColumnCount + TargetNotFoundRate;}
public TimeSpan TotalTime { get; set; }
}

Hashtable ht = new Hashtable();
Hashtable CreateColumnHash(SqlDataReader dr)
{
ht = new Hashtable();
for (int i = 0; i < dr.FieldCount; i++)
{
ht.Add(dr.GetName(i), dr.GetName(i));
}
return ht;
}
bool ValidateColumn(string ColumnName)
{
return ht.Contains(ColumnName);
}

Here is a one-liner LINQ version of the accepted answer:
Enumerable.Range(0, reader.FieldCount).Any(i => reader.GetName(i) == "COLUMN_NAME_GOES_HERE")

Here is the solution from Jasmine in one line... (one more, though simple!):
reader.GetSchemaTable().Select("ColumnName='MyCol'").Length > 0;

To keep your code robust and clean, use a single extension function, like this:
Public Module Extensions
<Extension()>
Public Function HasColumn(r As SqlDataReader, columnName As String) As Boolean
Return If(String.IsNullOrEmpty(columnName) OrElse r.FieldCount = 0, False, Enumerable.Range(0, r.FieldCount).Select(Function(i) r.GetName(i)).Contains(columnName, StringComparer.OrdinalIgnoreCase))
End Function
End Module

This code corrects the issues that Levitikon had with their code:
(adapted from: [1]: http://msdn.microsoft.com/en-us/library/system.data.datatablereader.getschematable.aspx)
public List<string> GetColumnNames(SqlDataReader r)
{
List<string> ColumnNames = new List<string>();
DataTable schemaTable = r.GetSchemaTable();
DataRow row = schemaTable.Rows[0];
foreach (DataColumn col in schemaTable.Columns)
{
if (col.ColumnName == "ColumnName")
{
ColumnNames.Add(row[col.Ordinal].ToString());
break;
}
}
return ColumnNames;
}
The reason for getting all of those useless column names and not the name of the column from your table...
Is because your are getting the name of schema column (i.e. the column names for the Schema table)
NOTE: this seems to only return the name of the first column...
EDIT: corrected code that returns the name of all columns, but you cannot use a SqlDataReader to do it
public List<string> ExecuteColumnNamesReader(string command, List<SqlParameter> Params)
{
List<string> ColumnNames = new List<string>();
SqlDataAdapter da = new SqlDataAdapter();
string connection = ""; // your sql connection string
SqlCommand sqlComm = new SqlCommand(command, connection);
foreach (SqlParameter p in Params) { sqlComm.Parameters.Add(p); }
da.SelectCommand = sqlComm;
DataTable dt = new DataTable();
da.Fill(dt);
DataRow row = dt.Rows[0];
for (int ordinal = 0; ordinal < dt.Columns.Count; ordinal++)
{
string column_name = dt.Columns[ordinal].ColumnName;
ColumnNames.Add(column_name);
}
return ColumnNames; // you can then call .Contains("name") on the returned collection
}

Neither did I get GetSchemaTable to work, until I found this way.
Basically I do this:
Dim myView As DataView = dr.GetSchemaTable().DefaultView
myView.RowFilter = "ColumnName = 'ColumnToBeChecked'"
If myView.Count > 0 AndAlso dr.GetOrdinal("ColumnToBeChecked") <> -1 Then
obj.ColumnToBeChecked = ColumnFromDb(dr, "ColumnToBeChecked")
End If

public static bool DataViewColumnExists(DataView dv, string columnName)
{
return DataTableColumnExists(dv.Table, columnName);
}
public static bool DataTableColumnExists(DataTable dt, string columnName)
{
string DebugTrace = "Utils::DataTableColumnExists(" + dt.ToString() + ")";
try
{
return dt.Columns.Contains(columnName);
}
catch (Exception ex)
{
throw new MyExceptionHandler(ex, DebugTrace);
}
}
Columns.Contains is case-insensitive btw.

My data access class needs to be backward compatible, so I might be trying to access a column in a release where it doesn't exist in the database yet. We have some rather large data sets being returned so I'm not a big fan of an extension method that has to iterate the DataReader column collection for each property.
I have a utility class that creates a private list of columns and then has a generic method that attempts to resolve a value based on a column name and output parameter type.
private List<string> _lstString;
public void GetValueByParameter<T>(IDataReader dr, string parameterName, out T returnValue)
{
returnValue = default(T);
if (!_lstString.Contains(parameterName))
{
Logger.Instance.LogVerbose(this, "missing parameter: " + parameterName);
return;
}
try
{
if (dr[parameterName] != null && [parameterName] != DBNull.Value)
returnValue = (T)dr[parameterName];
}
catch (Exception ex)
{
Logger.Instance.LogException(this, ex);
}
}
/// <summary>
/// Reset the global list of columns to reflect the fields in the IDataReader
/// </summary>
/// <param name="dr">The IDataReader being acted upon</param>
/// <param name="NextResult">Advances IDataReader to next result</param>
public void ResetSchemaTable(IDataReader dr, bool nextResult)
{
if (nextResult)
dr.NextResult();
_lstString = new List<string>();
using (DataTable dataTableSchema = dr.GetSchemaTable())
{
if (dataTableSchema != null)
{
foreach (DataRow row in dataTableSchema.Rows)
{
_lstString.Add(row[dataTableSchema.Columns["ColumnName"]].ToString());
}
}
}
}
Then I can just call my code like so
using (var dr = ExecuteReader(databaseCommand))
{
int? outInt;
string outString;
Utility.ResetSchemaTable(dr, false);
while (dr.Read())
{
Utility.GetValueByParameter(dr, "SomeColumn", out outInt);
if (outInt.HasValue) myIntField = outInt.Value;
}
Utility.ResetSchemaTable(dr, true);
while (dr.Read())
{
Utility.GetValueByParameter(dr, "AnotherColumn", out outString);
if (!string.IsNullOrEmpty(outString)) myIntField = outString;
}
}

The key to the whole problem is here:
if (-1 == index) {
throw ADP.IndexOutOfRange(fieldName);
}
If the referenced three lines (currently lines 72, 73, and 74) are taken out, then you can easily check for -1 in order to determine if the column doesn't exist.
The only way around this while ensuring native performance is to use a Reflection based implementation, like the following:
Usings:
using System;
using System.Data;
using System.Reflection;
using System.Data.SqlClient;
using System.Linq;
using System.Web.Compilation; // I'm not sure what the .NET Core equivalent to BuildManager.cs
The Reflection based extension method:
/// Gets the column ordinal, given the name of the column.
/// </summary>
/// <param name="reader"></param>
/// <param name="name">The name of the column.</param>
/// <returns> The zero-based column ordinal. -1 if the column does not exist.</returns>
public static int GetOrdinalSoft(this SqlDataReader reader, string name)
{
try
{
// Note that "Statistics" will not be accounted for in this implemenation
// If you have SqlConnection.StatisticsEnabled set to true (the default is false), you probably don't want to use this method
// All of the following logic is inspired by the actual implementation of the framework:
// https://referencesource.microsoft.com/#System.Data/fx/src/data/System/Data/SqlClient/SqlDataReader.cs,d66096b6f57cac74
if (name == null)
throw new ArgumentNullException("fieldName");
Type sqlDataReaderType = typeof(SqlDataReader);
object fieldNameLookup = sqlDataReaderType.GetField("_fieldNameLookup", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(reader);
Type fieldNameLookupType;
if (fieldNameLookup == null)
{
MethodInfo checkMetaDataIsReady = sqlDataReaderType.GetRuntimeMethods().First(x => x.Name == "CheckMetaDataIsReady" && x.GetParameters().Length == 0);
checkMetaDataIsReady.Invoke(reader, null);
fieldNameLookupType = BuildManager.GetType("System.Data.ProviderBase.FieldNameLookup", true, false);
ConstructorInfo ctor = fieldNameLookupType.GetConstructor(new[] { typeof(SqlDataReader), typeof(int) });
fieldNameLookup = ctor.Invoke(new object[] { reader, sqlDataReaderType.GetField("_defaultLCID", BindingFlags.NonPublic | BindingFlags.Instance).GetValue(reader) });
}
else
fieldNameLookupType = fieldNameLookup.GetType();
MethodInfo indexOf = fieldNameLookupType.GetMethod("IndexOf", BindingFlags.Public | BindingFlags.Instance, null, new Type[] { typeof(string) }, null);
return (int)indexOf.Invoke(fieldNameLookup, new object[] { name });
}
catch
{
// .NET Implemenation might have changed, revert back to the classic solution.
if (reader.FieldCount > 11) // Performance observation by b_levitt
{
try
{
return reader.GetOrdinal(name);
}
catch
{
return -1;
}
}
else
{
var exists = Enumerable.Range(0, reader.FieldCount).Any(i => string.Equals(reader.GetName(i), name, StringComparison.OrdinalIgnoreCase));
if (exists)
return reader.GetOrdinal(name);
else
return -1;
}
}
}

In your particular situation (all procedures has the same columns except one which has an additional one column), it will be better and faster to check the reader's FieldCount property to distinguish between them.
const int NormalColCount = .....
if(reader.FieldCount > NormalColCount)
{
// Do something special
}
You can also (for performance reasons) mix this solution with the solution iterating solution.

I would recommend using try{} catch{} for this simple issue. However, I would not recommend handling exception in catch.
try
{
if (string.IsNullOrEmpty(reader["Name"].ToString()))
{
name = reader["Name"].ToString();
}
}
catch
{
//Do nothing
}

This is a pretty old thread, but I wanted to provide my two cents.
The challenge with most of the proposed solutions is that it requires you to enumerate over all fields every time for every row for every column you're checking.
Others are using the GetSchemaTable method which is not globally supported.
Personally, I have no issue with throwing and catching exceptions to check if a field exists. In fact, I think it's probably the most straightforward solution from a programming perspective and the easiest to debug and create an extension for. I've noticed no negative performance hits on swallowing exceptions except where there is some other transaction involved or weird rollback logic.
Implementation using a try-catch block
using System;
using System.Collections.Generic;
using System.Data.SqlClient;
public class MyModel {
public int ID { get; set; }
public int UnknownColumn { get; set; }
}
public IEnumerable<MyModel> ReadData(SqlCommand command) {
using (SqlDataReader reader = command.ExecuteReader()) {
try {
while (reader.Read()) {
// init the row
MyModel row = new MyModel();
// bind the fields
row.ID = reader.IfDBNull("ID", row.ID);
row.UnknownColumn = reader.IfDBNull("UnknownColumn", row.UnknownColumn);
// return the row and move forward
yield return row;
}
} finally {
// technically the disposer should handle this for you
if (!reader.IsClosed) reader.Close();
}
}
}
// I use a variant of this class everywhere I go to help simplify data binding
public static class IDataReaderExtensions {
// clearly separate name to ensure I don't accidentally use the wrong method
public static T IfDBNull<T>(this IDataReader reader, string name, T defaultValue) {
T value;
try {
// attempt to read the value
// will throw IndexOutOfRangeException if not available
object objValue = reader[name];
// the value returned from SQL is NULL
if (Convert.IsDBNull(objValue)) {
// use the default value
objValue = defaultValue;
}
else if (typeof(T) == typeof(char)) {
// chars are returned from SQL as strings
string strValue = Convert.ToString(objValue);
if (strValue.Length > 0) objValue = strValue[0];
else objValue = defaultValue;
}
value = (T)objValue;
} catch (IndexOutOfRangeException) {
// field does not exist
value = #defaultValue;
} catch (InvalidCastException, ex) {
// The type we are attempting to bind to is not the same as the type returned from the database
// Personally, I want to know the field name that has the problem
throw new InvalidCastException(name, ex);
}
return value;
}
// clearly separate name to ensure I don't accidentally use the wrong method
// just overloads the other method so I don't need to pass in a default
public static T IfDBNull<T>(this IDataReader reader, string name) {
return IfDBNull<T>(reader, name, default(T));
}
}
If you want to avoid exception handling, I'd recommend saving your results to a HashSet<string> when you initialize your reader, then checking back to it for the columns you want. Alternatively for a micro-optimization, you can implement your columns as a Dictionary<string, int> to prevent a duplicate resolution from Name to ordinal by the SqlDataReader object.
Implementation using HashSet<string>
using System;
using System.Collections.Generic;
using System.Data.SqlClient;
public class MyModel {
public int ID { get; set; }
public int UnknownColumn { get; set; }
}
public IEnumerable<MyModel> ReadData(SqlCommand command) {
using (SqlDataReader reader = command.ExecuteReader()) {
try {
// first read
if (reader.Read()) {
// use whatever *IgnoreCase comparer that you're comfortable with
HashSet<string> columns = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
// init the columns HashSet<string, int>
for (int i = 0; i < reader.FieldCount; i++) {
string fieldName = reader.GetName(i);
columns.Add(fieldName);
}
// implemented as a do/while since we already read the first row
do {
// init a new instance of your class
MyModel row = new MyModel();
// check if column exists
if (columns.Contains("ID") &&
// ensure the value is not DBNull
!Convert.IsDBNull(reader["ID"])) {
// bind value
row.ID = (int)reader["ID"];
}
// check if column exists
if (columns.Contains("UnknownColumn") &&
// ensure the value is not DBNull
!Convert.IsDBNull(reader["UnknownColumn"])) {
// bind value
row.UnknownColumn = (int)reader["UnknownColumn"];
}
// return the row and move forward
yield return row;
} while (reader.Read());
}
} finally {
// technically the disposer should handle this for you
if (!reader.IsClosed) reader.Close();
}
}
}
Implementation using Dictionary<string, int>
using System;
using System.Collections.Generic;
using System.Data.SqlClient;
public class MyModel {
public int ID { get; set; }
public int UnknownColumn { get; set; }
}
public IEnumerable<MyModel> ReadData(SqlCommand command) {
using (SqlDataReader reader = command.ExecuteReader()) {
try {
// first read
if (reader.Read()) {
// use whatever *IgnoreCase comparer that you're comfortable with
Dictionary<string, int> columns = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);
// init the columns Dictionary<string, int>
for (int i = 0; i < reader.FieldCount; i++) {
string fieldName = reader.GetName(i);
columns[fieldName] = i;
}
// implemented as a do/while since we already read the first row
do {
// init a new instance of your class
MyModel row = new MyModel();
// stores the resolved ordinal from your dictionary
int ordinal;
// check if column exists
if (columns.TryGetValue("ID", out ordinal) &&
// ensure the value is not DBNull
!Convert.IsDBNull(reader[ordinal])) {
// bind value
row.ID = (int)reader[ordinal];
}
// check if column exists
if (columns.TryGetValue("UnknownColumn", out ordinal) &&
// ensure the value is not DBNull
!Convert.IsDBNull(reader[ordinal])) {
// bind value
row.UnknownColumn = (int)reader[ordinal];
}
// return the row and move forward
yield return row;
} while (reader.Read());
}
} finally {
// technically the disposer should handle this for you
if (!reader.IsClosed) reader.Close();
}
}
}

You can also call GetSchemaTable() on your DataReader if you want the list of columns and you don't want to have to get an exception...

Although there is no publicly exposed method, a method does exist in the internal class System.Data.ProviderBase.FieldNameLookup which SqlDataReader relies on.
In order to access it and get native performance, you must use the ILGenerator to create a method at runtime. The following code will give you direct access to int IndexOf(string fieldName) in the System.Data.ProviderBase.FieldNameLookup class as well as perform the book keeping that SqlDataReader.GetOrdinal()does so that there is no side effect. The generated code mirrors the existing SqlDataReader.GetOrdinal() except that it calls FieldNameLookup.IndexOf() instead of FieldNameLookup.GetOrdinal(). The GetOrdinal() method calls to the IndexOf() function and throws an exception if -1 is returned, so we bypass that behavior.
using System;
using System.Data;
using System.Data.SqlClient;
using System.Reflection;
using System.Reflection.Emit;
public static class SqlDataReaderExtensions {
private delegate int IndexOfDelegate(SqlDataReader reader, string name);
private static IndexOfDelegate IndexOf;
public static int GetColumnIndex(this SqlDataReader reader, string name) {
return name == null ? -1 : IndexOf(reader, name);
}
public static bool ContainsColumn(this SqlDataReader reader, string name) {
return name != null && IndexOf(reader, name) >= 0;
}
static SqlDataReaderExtensions() {
Type typeSqlDataReader = typeof(SqlDataReader);
Type typeSqlStatistics = typeSqlDataReader.Assembly.GetType("System.Data.SqlClient.SqlStatistics", true);
Type typeFieldNameLookup = typeSqlDataReader.Assembly.GetType("System.Data.ProviderBase.FieldNameLookup", true);
BindingFlags staticflags = BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.IgnoreCase | BindingFlags.Static;
BindingFlags instflags = BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.IgnoreCase | BindingFlags.Instance;
DynamicMethod dynmethod = new DynamicMethod("SqlDataReader_IndexOf", typeof(int), new Type[2]{ typeSqlDataReader, typeof(string) }, true);
ILGenerator gen = dynmethod.GetILGenerator();
gen.DeclareLocal(typeSqlStatistics);
gen.DeclareLocal(typeof(int));
// SqlStatistics statistics = (SqlStatistics) null;
gen.Emit(OpCodes.Ldnull);
gen.Emit(OpCodes.Stloc_0);
// try {
gen.BeginExceptionBlock();
// statistics = SqlStatistics.StartTimer(this.Statistics);
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Call, typeSqlDataReader.GetProperty("Statistics", instflags | BindingFlags.GetProperty, null, typeSqlStatistics, Type.EmptyTypes, null).GetMethod);
gen.Emit(OpCodes.Call, typeSqlStatistics.GetMethod("StartTimer", staticflags | BindingFlags.InvokeMethod, null, new Type[] { typeSqlStatistics }, null));
gen.Emit(OpCodes.Stloc_0); //statistics
// if(this._fieldNameLookup == null) {
Label branchTarget = gen.DefineLabel();
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Ldfld, typeSqlDataReader.GetField("_fieldNameLookup", instflags | BindingFlags.GetField));
gen.Emit(OpCodes.Brtrue_S, branchTarget);
// this.CheckMetaDataIsReady();
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Call, typeSqlDataReader.GetMethod("CheckMetaDataIsReady", instflags | BindingFlags.InvokeMethod, null, Type.EmptyTypes, null));
// this._fieldNameLookup = new FieldNameLookup((IDataRecord)this, this._defaultLCID);
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Ldfld, typeSqlDataReader.GetField("_defaultLCID", instflags | BindingFlags.GetField));
gen.Emit(OpCodes.Newobj, typeFieldNameLookup.GetConstructor(instflags, null, new Type[] { typeof(IDataReader), typeof(int) }, null));
gen.Emit(OpCodes.Stfld, typeSqlDataReader.GetField("_fieldNameLookup", instflags | BindingFlags.SetField));
// }
gen.MarkLabel(branchTarget);
gen.Emit(OpCodes.Ldarg_0); //this
gen.Emit(OpCodes.Ldfld, typeSqlDataReader.GetField("_fieldNameLookup", instflags | BindingFlags.GetField));
gen.Emit(OpCodes.Ldarg_1); //name
gen.Emit(OpCodes.Call, typeFieldNameLookup.GetMethod("IndexOf", instflags | BindingFlags.InvokeMethod, null, new Type[] { typeof(string) }, null));
gen.Emit(OpCodes.Stloc_1); //int output
Label leaveProtectedRegion = gen.DefineLabel();
gen.Emit(OpCodes.Leave_S, leaveProtectedRegion);
// } finally {
gen.BeginFaultBlock();
// SqlStatistics.StopTimer(statistics);
gen.Emit(OpCodes.Ldloc_0); //statistics
gen.Emit(OpCodes.Call, typeSqlStatistics.GetMethod("StopTimer", staticflags | BindingFlags.InvokeMethod, null, new Type[] { typeSqlStatistics }, null));
// }
gen.EndExceptionBlock();
gen.MarkLabel(leaveProtectedRegion);
gen.Emit(OpCodes.Ldloc_1);
gen.Emit(OpCodes.Ret);
IndexOf = (IndexOfDelegate)dynmethod.CreateDelegate(typeof(IndexOfDelegate));
}
}

Use:
if (dr.GetSchemaTable().Columns.Contains("accounttype"))
do something
else
do something
It probably would not be as efficient in a loop.

This works to me:
public static class DataRecordExtensions
{
public static bool HasColumn(IDataReader dataReader, string columnName)
{
dataReader.GetSchemaTable().DefaultView.RowFilter = $"ColumnName= '{columnName}'";
return (dataReader.GetSchemaTable().DefaultView.Count > 0);
}
}

Use:
if(Enumerable.Range(0,reader.FieldCount).Select(reader.GetName).Contains("columName"))
{
employee.EmployeeId= Utility.ConvertReaderToLong(reader["EmployeeId"]);
}
You can get more details from Can you get the column names from a SqlDataReader?.