ConditionalWeakTable<TKey, TValue>.Add Method

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.


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 MainClass
   string Name;

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

   public override string ToString()
      return this.Name;

// Define a class to contain information about each Example instance.
public class MainInfo
   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 Example
   private static BindingFlags flags = BindingFlags.Public | BindingFlags.Instance;

   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
      MainClass m1 = new MainClass("m1");
      MainClass m2 = new MainClass("m2");
      MainClass m3 = new MainClass("m3");

      MainInfo mInfo1 = new MainInfo();
      mInfo1.Name = m1.ToString();
      mInfo1.Methods = m1.GetType().GetMethods(flags).Length;
      mInfo1.Properties = m1.GetType().GetProperties(flags).Length;

      MainInfo mInfo3 = new MainInfo();
      mInfo3.Name = m3.ToString();
      mInfo3.Methods = m3.GetType().GetMethods(flags).Length;
      mInfo3.Properties = m3.GetType().GetProperties(flags).Length;

      var attached = new ConditionalWeakTable<MainClass, MainInfo>();
      MainInfo value = null;

      // Attach a property to m1 using the Add method, then retrieve it.
      attached.Add(m1, mInfo1);
      if (attached.TryGetValue(m1, out value))
         outputBlock.Text += String.Format("{0}, {1}", m1, value) + "\n";
         outputBlock.Text += String.Format("{0} does not have an attached property.", m1) + "\n";

      // Attempt to retrieve the value attached to m2.
      value = attached.GetValue(m2, Example.CreateAttachedValue);
      if (attached.TryGetValue(m2, out value))
         outputBlock.Text += String.Format("{0}, {1}", m2, value) + "\n";
         outputBlock.Text += String.Format("{0} does not have an attached property.", m2) + "\n";

      // Attempt to retrieve the value attached to m3.
      value = attached.GetOrCreateValue(m3);
      outputBlock.Text += String.Format("{0}, {1}", m3, value) + "\n";

   public static MainInfo CreateAttachedValue(MainClass m)
      MainInfo info = new MainInfo();
      info.Name = m.ToString();
      info.Methods = m.GetType().GetMethods(flags).Length;
      info.Properties = m.GetType().GetProperties(flags).Length;
      return info;
// The example displays the following output:
//       m1, m1: 4 Methods, 0 Properties
//       m2, m2: 4 Methods, 0 Properties
//       m3, : 0 Methods, 0 Properties


Supported in: 5, 4

  • SecurityCriticalAttribute 

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

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