ConditionalWeakTable<TKey, TValue>.Add Method (TKey, TValue)


Adds a key to the table.

Namespace:   System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)

public void Add(
	TKey key,
	TValue value


Type: TKey

The key to add. key represents the object to which the property is attached.

Type: TValue

The key's property value.

Exception Condition

key is null.


key already exists.

Every key in a ConditionalWeakTable<TKey, TValue> object must be unique. Keys are not unique if they refer to the same object (that is, if passing them as arguments to the Object.ReferenceEquals method returns true).

If the key is garbage-collected during the addition operation, the existing key/value pair is removed and the new key/value pair is added without an exception being thrown.

The following example defines a MainClass class and a MainInfo class, which provides information about the MainClass instance. The example then calls the Add method to add a MainClass object and its attached MainInfo object to a ConditionalWeakTable<TKey, TValue> table. The example also illustrates calls to the GetOrCreateValue and GetValue methods to add key/value pairs to the table, and to the TryGetValue method to retrieve the value of an existing key.

using System;
using System.Reflection;
using System.Runtime.CompilerServices;

public class Example
   string Name; 

   public Example(string name)
      this.Name = name;

   public override string ToString()
      return this.Name;

// Define a class to contain information about each Example instance.
public class ExampleInfo
   public string Name;
   public int Methods;
   public int Properties;

   public override string ToString()
      return String.Format("{0}: {1} Methods, {2} Properties", 
                           this.Name, this.Methods, this.Properties);

public class ExampleTest
   private static BindingFlags flags = BindingFlags.Public | BindingFlags.Instance;

   public static void Main()
      Example ex1 = new Example("ex1");
      Example ex2 = new Example("ex2");
      Example ex3 = new Example("ex3");

      ExampleInfo exInfo1 = new ExampleInfo(); 
      exInfo1.Name = ex1.ToString();
      exInfo1.Methods = ex1.GetType().GetMethods(flags).Length;
      exInfo1.Properties = ex1.GetType().GetProperties(flags).Length;

      ExampleInfo exInfo3 = new ExampleInfo(); 
      exInfo3.Name = ex3.ToString();
      exInfo3.Methods = ex3.GetType().GetMethods(flags).Length;
      exInfo3.Properties = ex3.GetType().GetProperties(flags).Length;

      var attached = new ConditionalWeakTable<Example, ExampleInfo>();
      ExampleInfo value = null;

      // Attach a property to ex1 using the Add method, then retrieve it.
      attached.Add(ex1, exInfo1);
      if (attached.TryGetValue(ex1, out value))
         Console.WriteLine("{0}, {1}", ex1, value);
         Console.WriteLine("{0} does not have an attached property.", ex1);

      // Attempt to retrieve the value attached to ex2.
      value = attached.GetValue(ex2, ExampleTest.CreateAttachedValue);      
      if (attached.TryGetValue(ex2, out value))
         Console.WriteLine("{0}, {1}", ex2, value);
         Console.WriteLine("{0} does not have an attached property.", ex2);

      // Attempt to retrieve the value attached to ex3.
      value = attached.GetOrCreateValue(ex3);
      Console.WriteLine("{0}, {1}", ex3, value);

   public static ExampleInfo CreateAttachedValue(Example ex)
      ExampleInfo info = new ExampleInfo();
      info.Name = ex.ToString();
      info.Methods = ex.GetType().GetMethods(flags).Length;
      info.Properties = ex.GetType().GetProperties(flags).Length;
      return info;
// The example displays the following output:
//       ex1, ex1: 4 Methods, 0 Properties
//       ex2, ex2: 4 Methods, 0 Properties
//       ex3, : 0 Methods, 0 Properties


requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Available since 4.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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