I have a DataTable with multiple rows. I'm using a foreach loop to loop through each item and return the name. This is returning the same (1st) value for each row. What have I done wrong?
DataTable table = new DataTable();
table.Columns.Add("tag", typeof(string));
string name = hfSelected.Value;
string[] names = name.Split(',');
for (int i = 0; i < names.Length; i++)
table.Rows.Add(new object[] { names[i] });
DataRow row = table.Rows[0];
foreach (var item in table.Rows)
{
Value = row["tag"].ToString() // this is returning the same value for both items in the table.
}
In a comment you mentioned that you get the error:
cannot apply indexing with [] to an expression of type object
when trying to access item["tag"] in the foreach loop.
You need to explicitly declare the DataRow in the foreach.
// declare DataRow here, not var
foreach (DataRow item in table.Rows)
{
// use item here
Value = item["tag"].ToString(); // use += to concatenate string
}
The reason is that the DataRowCollection implements a non-generic IEnumerable so you index an object instead of DataRow. The solution above casts to a DataRow.
I would recommend looking at the Field<T>() and AsEnumerable() methods from System.Data.DataSetExtensions. AsEnumerable() returns an IEnumerable<DataRow>. Field() provides strongly typed access to the values (ie it casts/converts the types for you).
Then you can do:
foreach (var item in table.AsEnumerable())
{
// item is a DataRow here
var myString = item.Field<string>("tag"); // gets string
// you can also do
var myInt = item.Field<int>("Id"); // gets int
var myDate = item.Field<DateTime?>("Date"); // gets nullable DateTime?
var myValue = item.Field<decimal>("Price"); // gets decimal
}
Carl is correct, this is producing the same output, because inside the iteration, you use the same row, all the time. You should use 'item', instead of 'row' there (you don't need 'row' at all).
The exception you receive is because you declared 'item' with a dynamic type, it's
foreach (var item in table.Rows)
You can try
foreach (DataRow item in table.Rows)
this way, you'll be able to get the column info.
your iteration seems to be using the same 'row' variable instead of the 'item' variable you defined in the foreach statement.
Related
I am working on a "ToDataTable()" that I can use on IEnumerable. Lots of examples of that, such as here: Convert IEnumerable to DataTable. DataTables are desirable for me because I am coming from a Foxpro realm where we are used to cursors. So, I dig DataTables because they are relatively simple, easy to work with, and can house decent column meta-data. And I've already got a bunch of code that can display them nicely (with sort and filter grid headers), export to Excel, etc.
Someone made a really good comment on the thread I linked to above: "This doesn't work if the type is string as the properties are chars and length but the get value then tries to put the full string into the char column."
Problem is, certain things fit the extension method call (like a one-dimensional array of strings) but then throw errors when the conversion takes place. I wrote an extension method that adds IsValueType() to the mix, but the problem is that returns false for strings. Finally, I added two kludges: a parameter to force the item in the sequence to be treated as a value, and to treat the item in the sequence as a value if it is found to be of type "String".
Is there a better way of doing this, or are strings just an odd man out due to their unique breed of typing and reflection results? The following code works with every IEnumerable I can think of, and it works with one-dimensional arrays for all DataTable column types except for string (well, it works for strings with the hard-coded kludge, but would error out otherwise). It's not a big deal to hard-code the string scenario, I just wish there was a more elegant way to do this. Any thoughts?
public static DataTable ToDataTable<T>(this IEnumerable<T> items, string tableName = "", bool treatItemAsValue = false)
{
// We want a single extension method that can take in an enumerable sequence (such as a LINQ query)
// and return the result as a DataTable. We want this to be a one stop shop for converting
// various objects into DataTable format, as DataTables are a nice parallel to Foxpro cursors.
if (items == null) { return null; }
Type itemType = typeof(T);
bool typeIsNullable = itemType.IsGenericType && typeof(T).GetGenericTypeDefinition().Equals(typeof(Nullable<>));
string itemTypeName = "";
bool typeIsValue = false;
Type itemUnderlyingType = itemType;
if (typeIsNullable)
{
// Type of enumerable item is nullable, so we need to find its base type.
itemUnderlyingType = Nullable.GetUnderlyingType(itemType);
}
typeIsValue = itemUnderlyingType.IsValueType;
itemTypeName = itemUnderlyingType.Name;
DataTable dt = new DataTable();
DataColumn col = null;
if ((treatItemAsValue) || (itemTypeName == "String"))
{
// We have been asked to treat the item in the sequence as a value, of the items
// in the sequence are strings. Strings are NOT considered a value type in regards
// to IsValueType(), but when item values are assessed, it will be the value
// of the string that tries to pull in.
typeIsValue = true;
}
if (itemTypeName == "DataRow")
{
// Special case. If our enumerable type is DataRow, then we can utilize a more appropriate
// (built-in) extension method to convert enumerable DataRows to a DataTable.
dt = ((IEnumerable<DataRow>)items).CopyToDataTable();
}
else
{
// We must have an enumerable sequence/collection of some other type, possibly anonymous.
// Get properties of the enumerable to add as columns to the data table.
if (typeIsValue)
{
// Our enumerable items are of a value type (e.g. integers in a one-dimensional array).
col = dt.Columns.Add();
col.AllowDBNull = typeIsNullable;
col.ColumnName = itemTypeName;
col.DataType = itemUnderlyingType;
// Now walk through the enumeration and add rows to our data table (single values).
foreach (var item in items)
{
dt.Rows.Add(item);
}
}
else
{
// The type should be something we can walk through the properties of in order to
// generate properly named and typed columns of our DataTable.
PropertyInfo[] props = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance);
foreach (var prop in props)
{
Type propType = prop.PropertyType;
// Is it a nullable type? Get the underlying type.
if (propType.IsGenericType && propType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
{
propType = new NullableConverter(propType).UnderlyingType;
}
dt.Columns.Add(prop.Name, propType);
}
// Now walk through the enumeration and add rows to our data table.
foreach (var item in items)
{
var values = new object[props.Length];
for (int i = 0; i < props.Length; i++)
{
values[i] = props[i].GetValue(item, null);
}
dt.Rows.Add(values);
}
}
}
// Give the DataTable a reasonable name.
if (tableName.Length == 0)
{
if (typeof(T).IsAnonymous())
{
// Anonymous types have really goofy names, so there is no use using that as table name.
tableName = "Anonymous";
}
else
{
// This is NOT an anonymous type, so we can use the type name as table name.
tableName = typeof(T).Name;
}
}
return dt;
}
Well, no responses except a comment that pretty much missed the point, so I'll just post what I ended up with. This final version also accounts for items with no properties (such as "Object" itself), and does some better handing of null items/values:
public static DataTable ToDataTable<T>(this IEnumerable<T> items, string tableName = "", bool treatItemAsValue = false)
{
// We want a single extension method that can take in an enumerable sequence (such as a LINQ query)
// and return the result as a DataTable. We want this to be a one stop shop for converting
// various objects into DataTable format, as DataTables are a nice parallel to Foxpro cursors.
if (items == null) { return null; }
Type itemType = typeof(T);
bool typeIsNullable = itemType.IsGenericType && typeof(T).GetGenericTypeDefinition().Equals(typeof(Nullable<>));
string itemTypeName = "";
bool typeIsValue = false;
Type itemUnderlyingType = itemType;
if (typeIsNullable)
{
// Type of enumerable item is nullable, so we need to find its base type.
itemUnderlyingType = Nullable.GetUnderlyingType(itemType);
}
typeIsValue = itemUnderlyingType.IsValueType;
itemTypeName = itemUnderlyingType.Name;
DataTable dt = new DataTable();
DataColumn col = null;
PropertyInfo[] props = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance);
if ((treatItemAsValue) || (itemTypeName == "String") || (props.Length == 0))
{
// We have been asked to treat the item in the sequence as a value, or the items
// in the sequence is a string which cannot be "flattened" properly by analyzing properties.
// OR, the type has no properties to put on display, so we should just use the item directly.
// (like the base "Object" type).
typeIsValue = true;
}
if (itemTypeName == "DataRow")
{
// Special case. If our enumerable type is DataRow, then we can utilize a more appropriate
// (built-in) extension method to convert enumerable DataRows to a DataTable.
dt = ((IEnumerable<DataRow>)items).CopyToDataTable();
}
else
{
// We must have an enumerable sequence/collection of some other type, possibly anonymous.
// Get properties of the enumerable to add as columns to the data table.
if (typeIsValue)
{
// Our enumerable items are of a value type (e.g. integers in a one-dimensional array).
col = dt.Columns.Add();
// Whether or not the type is nullable, the value might be null (e.g. for type "Object").
col.AllowDBNull = true;
col.ColumnName = itemTypeName;
col.DataType = itemUnderlyingType;
// Now walk through the enumeration and add rows to our data table (single values).
foreach (var item in items)
{
dt.Rows.Add(item);
}
}
else
{
// The type should be something we can walk through the properties of in order to
// generate properly named and typed columns of our DataTable.
foreach (var prop in props)
{
Type propType = prop.PropertyType;
// Is it a nullable type? Get the underlying type.
if (propType.IsGenericType && propType.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
{
propType = new NullableConverter(propType).UnderlyingType;
}
dt.Columns.Add(prop.Name, propType);
}
// Now walk through the enumeration and add rows to our data table.
foreach (var item in items)
{
if (item != null)
{
// Can only add an item as a row if it is not null.
var values = new object[props.Length];
for (int i = 0; i < props.Length; i++)
{
values[i] = props[i].GetValue(item, null);
}
dt.Rows.Add(values);
}
}
}
}
// Give the DataTable a reasonable name.
if (tableName.Length == 0)
{
if (typeof(T).IsAnonymous())
{
// Anonymous types have really goofy names, so there is no use using that as table name.
tableName = "Anonymous";
}
else
{
// This is NOT an anonymous type, so we can use the type name as table name.
tableName = typeof(T).Name;
}
}
return dt;
}
Hope someone finds this useful...
foreach (var row in table.Rows)
{
DoSomethingWith(row);
}
Assuming that I'm working with a standard System.Data.DataTable (which has a collection of System.Data.DataRow objects), the variable 'row' above resolves as an object type, not a System.Data.DataRow.
foreach (DataRow row in table.Rows)
{
DoSomethingWith(row);
}
Works as I would expect. Is there a particular reason for this?
Thanks.
That's because Rows is DataRowCollection, which in turn is IEnumerable and not IEnumerable<DataRow>, which means that type inferred will be object.
When you explicitly state type in foreach, you instruct c# to add cast to each call, which is why it works.
An implicit cast happens. Also note that an InvalidCastException can be thrown if the cast isn't possible.
table.Rows is a DataRowCollection which is IEnumberable ( and not IEnumerable<T>, T being DataRow), so it is not strongly typed to a DataRow, but a object i.e it is a collection of objects.
There is a DataTable extensions which you can use though - http://msdn.microsoft.com/en-us/library/system.data.datatableextensions.asenumerable.aspx
foreach (var row in table.AsEnumerable())
{
}
Try this:
System.Data.DataTable dt = new System.Data.DataTable();
foreach (var row in dt.Rows.Cast<System.Data.DataRow>())
{
}
To use Rows.Cast you have to use System.Linq.
I am a big fan of using 'var' in C# rather than typing out all the data types. I find it annoying that
var dt = CreateAndPopulateDataTable() // Immaterial
foreach (var row in dt.Rows)
returns the 'row' as an object variable rather than a DataRow variable. I know I can cast it later or just use 'DataRow' rather than 'var'. Are those my only two options?
You can get an IEnumerable<DataRow> by using the AsEnumerable() method against the table.
foreach (var row in dt.AsEnumerable())
However, in this case, you are hardly saving characters verus simply going ahead and specifying the type.
foreach (DataRow row in dt.Rows)
I have the below function
public static DataTable ToTable<T>(this IEnumerable<T> listItem)
{
//Return null if the list is empty
if (listItem == null || listItem.Count() == 0) return null;
//Gets the type of the object
var listType = listItem.First().GetType();
//Initialize a new datatable
var dataTable = new DataTable(listType.Name);
//Create the datatable column names and types
listType.GetProperties().ToList().ForEach(col => dataTable.Columns.Add(col.Name, col.PropertyType));
//Get the datatable column names
var dataTableColumnNames = dataTable.GetDatatableColumnNames();
listItem.ToList().ForEach(item =>
{
//create a new datarow
var dataRow = dataTable.NewRow();
dataTableColumnNames
.Where(propName => listType.GetProperty(propName) != null)
.ToList()
.ForEach(columnName =>
//Exception happens here in the next line
dataRow[columnName] = listType.GetProperty(columnName).GetValue(item, null));
//Add the row to the data table
dataTable.Rows.Add(dataRow);
});
//Commit the changes to the datatable
dataTable.AcceptChanges();
return dataTable;
}
It works great for dictionary object and generic list as List<MyClass> .. but not for
List<string> or string[].
For those I am getting an exception as Parameter count mismatch.
The error is coming at
dataRow[columnName] = listType.GetProperty(columnName).GetValue(item, null));
What is the mistake that is happening?
Please help
Here's the deal. The index operator is actually considered a property when using reflection, hence parameter count mismatch.
If you break into your code and check the properties that are actually being enumerated by GetProperties(), you'll see the "Chars" property. That's the String's index operator. Since you didn't provide an index, you're getting a Parameter Count Mismatch error.
In essence, I assume string doesn't have any properties you want to put in your data table, but rather the string instance IS what you want to put in the data table.
You could create a model to store the string in, with the string as a property on the model, then the string would be stored with your current code. Otherwise, you will need to rethink your table generation algorithm for primitive types.
I hope this helps :)
Because one of the public properties of string is an indexer and you pass null as the index value. So you effectively end up doing this: string[null] which ends up in an exception.
I haven't verified this as I don't have VS available right now so I might be wrong but I'm pretty sure that's the problem.
Update: This question answers how you detect an indexed property: C# Reflection Indexed Properties
I have a data table and I want to select all distinct names from the result. I wrote following linq query for it.
var distinctRows = (from DataRow myDataRow in myDataTable.Rows
select new { col1 = myDataRow ["Name"]}).Distinct();
Now how can I iterate through distinctRows? Seems like I cannot do foreach(DataRow Row in distinctRows), It gives me "Cannot convert type 'AnonymousType#1' to 'System.Data.DataRow'" error
Since you're only selecting one field, you don't need an anonymous type here. Just select the names and then iterate over the distinct ones. To wit:
var distinctNames = (from DataRow myDataRow in myDataTable.Rows
select myDataRow.Field<string>("Name")
).Distinct();
foreach(var name in distinctNames) {
Console.WriteLine(name);
}
Note that the error makes it very clear what the problem is here. You are trying to convert an instance of an anonymous type to an instance of DataRow and that is impossible. Without changing your code, you could iterate this as
foreach(var item in distinctRows) {
Console.WriteLine((string)item.col1);
}
But I would change this as per the above as you don't need the anonymous type and your variable names and field names are poor.
Those aren't DataRows; they're anonymous objects.
To loop through them, you need to declare the variable using the var keyword.
However, there's no point in the anonymous type in the first place.
You can change your query to select myDataRow.Field<string>("Name") to get a set of strings.
You can use the keyword var to refer to anonymous types (which is what you're returning an IEnumerable<> of).
foreach(var row in distinctRows)
{
// do something with each anonymous type instance
}
Since you're only returning anonymous types with one string property however, you may as well project an IEnumerable<string>
That's because myDataRow["Name"] doesn't return a DataRow. Try
foreach(var item in distinctRows) {}
That's because the return value isn't a DataRow. It's an ad-hoc type that containes the property col1.
To build on SLaks answer . . .
var distinctRows = (from DataRow myDataRow in myDataTable.Rows
select new { col1 = myDataRow ["Name"]}).Distinct();
foreach(var row in distinctRows)
{
System.Console.Writeline(row.col1); //should work fine
}
The problem here is that you're selecting a new anonymous type by doing select new { col1 = myDataRow ["Name"]} and not the actual row itself. So when you try to iterate this as DataRow, it will error out because your anonymous type being selected is not of type DataRow.
If you want to be able to select a whole data row and not just the name field, you will need to implement a custom IEqualityComparer for data row to pass to the Distinct() extension method.
An example would be:
public class NameComparer : IEqualityComparer<DataRow>
{
public bool Equals(DataRow left, DataRow right)
{
return left.Field<string>("Name").Equals(right.Field<string>("Name"));
}
public int GetHashCode(DataRow obj)
{
return obj.ToString().GetHashCode();
}
}
Then using it:
var distinctRows = (from DataRow myDataRow in myDataTable.Rows
select myDataRow).Distinct(new NameComparer());
You can also foreach them but first you had to List them as the following :
List<string> rslt =(from DataRow myDataRow in myDataTable.Rows
select new { col1 = myDataRow ["Name"].ToString()}).Distinct().ToList();
foreach(string str in rlst)
{}
Hope this helped