i have an array of custom objects. i'd like to be able to reference this array by a particular data member, for instance myArrary["Item1"]
"Item1" is actually the value stored in the Name property of this custom type and I can write a predicate to mark the appropriate array item. However I am unclear as to how to let the array know i'd like to use this predicate to find the array item.
I'd like to just use a dictionary or hashtable or NameValuePair for this array, and get around this whole problem but it's generated and it must remain as CustomObj[]. i'm also trying to avoid loading a dictionary from this array as it's going to happen many times and there could be many objects in it.
For clarification
myArray[5] = new CustomObj() // easy!
myArray["ItemName"] = new CustomObj(); // how to do this?
Can the above be done? I'm really just looking for something similar to how DataRow.Columns["MyColumnName"] works
Thanks for the advice.
What you really want is an OrderedDictionary. The version that .NET provides in System.Collections.Specialized is not generic - however there is a generic version on CodeProject that you could use. Internally, this is really just a hashtable married to a list ... but it is exposed in a uniform manner.
If you really want to avoid using a dictionary - you're going to have to live with O(n) lookup performance for an item by key. In that case, stick with an array or list and just use the LINQ Where() method to lookup a value. You can use either First() or Single() depending on whether duplicate entries are expected.
var myArrayOfCustom = ...
var item = myArrayOfCustom.Where( x => x.Name = "yourSearchValue" ).First();
It's easy enough to wrap this functionality into a class so that external consumers are not burdened by this knowledge, and can use simple indexers to access the data. You could then add features like memoization if you expect the same values are going to be accessed frequently. In this way you could amortize the cost of building the underlying lookup dictionary over multiple accesses.
If you do not want to use "Dictionary", then you should create class "myArrary" with data mass storage functionality and add indexers of type "int" for index access and of type "string" for associative access.
public CustomObj this [string index]
{
get
{
return data[searchIdxByName(index)];
}
set
{
data[searchIdxByName(index)] = value;
}
}
First link in google for indexers is: http://www.csharphelp.com/2006/04/c-indexers/
you could use a dictionary for this, although it might not be the best solution in the world this is the first i came up with.
Dictionary<string, int> d = new Dictionary<string, int>();
d.Add("cat", 2);
d.Add("dog", 1);
d.Add("llama", 0);
d.Add("iguana", -1);
the ints could be objects, what you like :)
http://dotnetperls.com/dictionary-keys
Perhaps OrderedDictionary is what you're looking for.
you can use HashTable ;
System.Collections.Hashtable o_Hash_Table = new Hashtable();
o_Hash_Table.Add("Key", "Value");
There is a class in the System.Collections namespace called Dictionary<K,V> that you should use.
var d = new Dictionary<string, MyObj>();
MyObj o = d["a string variable"];
Another way would be to code two methods/a property:
public MyObj this[string index]
{
get
{
foreach (var o in My_Enumerable)
{
if (o.Name == index)
{
return o;
}
}
}
set
{
foreach (var o in My_Enumerable)
{
if (o.Name == index)
{
var i = My_Enumerable.IndexOf(0);
My_Enumerable.Remove(0);
My_Enumerable.Add(value);
}
}
}
}
I hope it helps!
It depends on the collection, some collections allow accessing by name and some don't. Accessing with strings is only meaningful when the collection has data stored, the column collection identifies columns by their name, thus allowing you to select a column by its name. In a normal array this would not work because items are only identified by their index number.
My best recommendation, if you can't change it to use a dictionary, is to either use a Linq expression:
var item1 = myArray.Where(x => x.Name == "Item1").FirstOrDefault();
or, make an extension method that uses a linq expression:
public static class CustomObjExtensions
{
public static CustomObj Get(this CustomObj[] Array, string Name)
{
Array.Where(x => x.Name == Name).FirstOrDefault();
}
}
then in your app:
var item2 = myArray.Get("Item2");
Note however that performance wouldn't be as good as using a dictionary, since behind the scenes .NET will just loop through the list until it finds a match, so if your list isn't going to change frequently, then you could just make a Dictionary instead.
I have two ideas:
1) I'm not sure you're aware but you can copy dictionary objects to an array like so:
Dictionary dict = new Dictionary();
dict.Add("tesT",40);
int[] myints = new int[dict.Count];
dict.Values.CopyTo(myints, 0);
This might allow you to use a Dictionary for everything while still keeping the output as an array.
2) You could also actually create a DataTable programmatically if that's the exact functionality you want:
DataTable dt = new DataTable();
DataColumn dc1 = new DataColumn("ID", typeof(int));
DataColumn dc2 = new DataColumn("Name", typeof(string));
dt.Columns.Add(dc1);
dt.Columns.Add(dc2);
DataRow row = dt.NewRow();
row["ID"] = 100;
row["Name"] = "Test";
dt.Rows.Add(row);
You could also create this outside of the method so you don't have to make the table over again every time.
Related
I have a class that is instantiated at the beginning of each iteration of a loop. Inside the loop, it's attributes need to be populated with the row values of a table returned by a stored procedure. As I have to iterate through each column of every row, in order to know which attribute of the class needs to be assigned a value and when, I have a dictionary that maps the column names to an index. This index refers to a position in a list that stores an attribute of an instance of the class:
while (reader.Read() && reader.HasRows)
{
Subscription subscription = new Subscription();
List<string> subscrData = new List<string>
{
subscription.attr1,
subscription.attr2,
subscription.attr3,
subscription.attr4
}
Dictionary<string, int> columnDict = new Dictionary<string, int>
{
{"attr1": 0},
{"attr2":1},
{"attr3":2},
{"attr4":3}
}
foreach (string colName in columnDict.Keys)
{
if (reader.GetSchemaTable().Columns[colName] == null)
subscrData[columnDict[colName]] = "null";
else
{
subscrData[columnDict[colName]] = reader[colName].ToString();
nullsReturned = false;
}
}
I'm probably coming at this from more of a C++ approach as with that you could store references to the class instance an modify its attributes, but this doesn't work with C# because lists store the values.
How can I restructure this code so that I can modify the actual attributes of the class instance while still being able to check if each column returned from the stored procedure is not null?
You don't need the list for this case. You either want to add a method like setAttribute(string attributeName) to your class (and within it build a switch/case to modify the given attribute); or, use reflection to change an instance field given its name.
I agree with Hasan. But just for your information: to implement your approach you could make use of Lambda expressions to keep track of the references to your properties (= the attributes).
Something like this would work:
Subscription subscription = new Subscription();
List<Expression<Func<Subscription, string>>> subscrData = new List<Expression<Func<Subscription, string>>>
{
a => a.attr1,
a => a.attr2,
a => a.attr3,
a => a.attr4,
};
//E.g. To update attribute 3 you can do this:
var prop = (PropertyInfo)((MemberExpression)subscrData[2].Body).Member;
prop.SetValue(subscription, "test string", null);
What I am trying to achieve in C# without using LINQ is to subset a list of custom-class objects based on the value of one of the parameters of these objects.
Let's say the declaration of my list is the following:
List<MyCustom> listofobj = new List<MyCustom>();
Also, assume that the custom-class object "MyCustom" can return two parameters: MyCustom.name and MyCustom.age
Is there a way trough which I can retrieve and save into a temporary new list (let's call it "templist") the subset of the original list (i.e. "listofobj") formed by all its MyCustom objects that have "age" parameter greater than 30? Thanks!
Sure. You can reinvent LINQ:
IEnumerable<MyCustom> WhereByAge(IEnumerable<MyCustom> source, int age)
{
foreach (MyCustom myCustom in source)
{
if (myCustom.Age > age)
{
yield return myCustom;
}
}
}
then:
List<MyCustom> filteredList = new List(WhereByAge(listofobj, 30));
But why? I strongly recommend that if you want LINQ-like behavior, just use LINQ.
This is how you get a subset of original list based on parameters without linq.
List<MyCustom> listofobj = new List<MyCustom>();
List<MyCustom> templist = new List<MyCustom>();
foreach(var obj in listofobj)
{
if(obj.Age > 30)
{
templist.Add(obj);
}
}
You can also use this Linq.
templist = listofobj.Where(obj => obj.Age > 30).ToList();
Linq is slower than a normal code in general. but this should not be problem if this is not hot path in your code. performance difference is negligible.
My data source could have duplicate keys with values.
typeA : 1
typeB : 2
typeA : 11
I chose to use NameValueCollection as it enables entering duplicate keys.
I want to remove specific key\value pair from the collection, but NameValueCollection.Remove(key) removes all values associated with the specified key.
Is there a way to remove single key\value pair from a NameValueCollection,
OR
Is there a better collection in C# that fits my data
[EDIT 1]
First, thanks for all the answers :)
I think I should have mentioned that my data source is XML.
I used System.Xml.Linq.XDocument to query for type and also it was handy to remove a particular value.
Now, my question is, for large size data, is using XDocument a good choice considering the performance?
If not what are other alternatives (maybe back to NameValueCollection and using one of the techniques mentioned to remove data)
The idea of storing multiple values with the same key is somehow strange. But I think you can retrieve all values using GetValues then remove the one you don't need and put them back using Set and then subsequent Add methods. You can make a separate extension method method for this.
NameValueCollection doesn't really allow to have multiple entries with the same key. It merely concatenates the new values of existing keys into a comma separated list of values (see NameValueCollection.Add.
So there really is just a single value per key. You could conceivably get the value split them on ',' and remove the offending value.
Edit: #ElDog is correct, there is a GetValues method which does this for you so no need to split.
A better option I think would be to use Dictionary<string, IList<int>> or Dictionary<string, ISet<int>> to store the values as discrete erm, values
You may convert it to Hashtable
var x = new NameValueCollection();
x.Add("a", "1");
x.Add("b", "2");
x.Add("a", "1");
var y = x.AllKeys.ToDictionary(k => k, k=>x[k]);
make your own method, it works for me --
public static void Remove<TKey,TValue>(
this List<KeyValuePair<TKey,TValue>> list,
TKey key,
TValue value) {
return list.Remove(new KeyValuePair<TKey,TValue>(key,value));
}
then call it on list as --
list.Remove(key,value); //Pass the key value...
Perhaps not the best way, but....
public class SingleType
{
public string Name;
public int Value;
}
List<SingleType> typeList = new List<SingleType>();
typeList.Add (new SingleType { Name = "TypeA", Value = 1 });
typeList.Add (new SingleType { Name = "TypeA", Value = 3 });
typeList.Remove (typeList.Where (t => t.Name == "TypeA" && t.Value == 1).Single());
You can use the Dictionary collection instead:
Dictionary<string, int> dictionary = new Dictionary<string, int>();
dictionary.Add("typeA", 1);
dictionary.Add("typeB", 1);
When you try to insert type: 11 it will throw exception as Key already exists. So you can enter a new key to insert this data.
Refer this Tutorial for further help.
I have a Collection object (based on System.Collections.CollectionBase) but to access the values of objects within that collection, I have to use the index currently. Is it possible to get the values based on the name of the object within the collection?
For example, instead of...
MyCollection[0].Value
... how can I do something along the lines of:
MyCollection["Birthday"].Value
In order to do this you would need to have a Dictionary<string,object>. Unfortunately collections only allow random access by index.
You could do something like this:
var item = MyCollection
.Where(x => x.SomeProp == "Birthday")
.FirstOrDefault();
// careful - item could be null here
var value = item.Value;
But this will be nowhere near as efficient as random access by index.
You could use a Dictionary<TKey, TValue> which allows you to access its element by a key. So if the key in your example is a string you could use Dictionary<string, TValue>.
Why do you think objects in a collection have names? They don't. What you can do is use a Dictionary<String, SomethingElse> to enable your syntax.
As others has said, you need a Dictionary<> to do that. If you cannot change the code that provides the collection, you can use LINQ's ToDictionary() method to convert it to a dictionary yourself:
var dict = MyCollection.ToDictionary(obj => obj.Name);
From there on, you can do:
var value = dict["Birthday"].Value;
You could use the this[] accessor
public Item this[string name]
{
get
{
// iterate through the elements of the collection
//and return the one that matches with name
}
}
Have this getter property on your MyCollectionClass
One workaround could be
private const int BIRTHDAY = 0;
var value = MyCollection["Birthday"].Value;
I am getting a datatable with customer data from a MySql databaes and a customer object from a web service.
I want to compare every value in the datatable with the values in the object and if there is one field that differs I want to perfrom some tasks.
I know I can get the values from the datatable with:
string mCompanyName = row["Company Name"].ToString();
string mCreatedDate = row["Created Date"].Tostring();
//etc..
Then I get the values from the web service
string wsCompanyName = customer.companyName;
string wsCreatedDate = customer.createdDate;
There are about 50 fields and doing
if( mCompanyName != wsCompanyName & mCreatedDate != wsCreatedDate and so on..) (or similar)
{
//Do something
}
seems to be a bit tedious and not very nice so how should I perform this? Is there a much better way to chuck it into a list and use some fancy LINQ?
Thanks in advance.
For cases like this I sometimes put them ("the objects") in something IEnumerable (make sure to "line them up") and use the SequenceEqual extension method. It performs standard Equals()'ity and is "cheap enough for my usage".
For instance:
var equal = (new object[] { row["A"], row["B"] })
.SequenceEqual(new object[] { x.A, x.B });
This requires LINQ, of course.
I'd put them in a Dictionary and search that way:
Dictionary<string, string> mData = new Dictionary<string, string>();
mData.Add("Company Name", row["Company Name"].ToString());
Dictionary<string, string> wsData = new Dictionary<string, string>();
wsData.Add("Company Name", customer.CompanyName);
Then loop through:
foreach (KeyValuePair<string, string> pair in mData)
{
if (wsData[pair.Key] == pair.Value)
{
// Do something
}
}
This way, for every entry in mData (the data from your database), it will look for an entry in wsData with the same name.
I wouldn't create individual variables for each piece of data. It would be difficult to maintain, and would not scale well (lots of copy and pastes).
I think this might help you, but it needs to modify this to use in your scenerio stackoverflow link