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ConditionalWeakTable<TKey, TValue>.GetOrCreateValue Method

Atomically searches for a specified key in the table and returns the corresponding value. If the key does not exist in the table, the method invokes the default constructor of the class that represents the table's value to create a value that is bound to the specified key.

Namespace:  System.Runtime.CompilerServices
Assembly:  mscorlib (in mscorlib.dll)
public TValue GetOrCreateValue(
	TKey key
)

Parameters

key
Type: TKey
The key to search for. key represents the object to which the property is attached.

Return Value

Type: TValue
The value that corresponds to key, if key already exists in the table; otherwise, a new value created by the default constructor of the class defined by the TValue generic type parameter.
ExceptionCondition
ArgumentNullException

key is null.

MissingMethodException

The class that represents the table's value does not define a default constructor.

If key does not exist in the table, the method adds it, along with the object that is instantiated by calling the default constructor of the class defined by the TValue generic type parameter. If the TValue class has no default constructor, a MissingMethodException is thrown.

This is the recommended method of retrieving an existing value or adding a new value to the ConditionalWeakTable<TKey, TValue> table if the class of the table's value defines a default constructor. If it does not define a default constructor, you can instead call the GetValue method, which relies on a callback-provided method to instantiate the object representing the table's value

To retrieve the value of an existing key without adding the key/value pair if the key is not found in the table, call the TryGetValue method.

The following example defines a MainClass class and a MainInfo class, which provides information about the MainClass instance. The example calls the GetOrCreateValue method to add a MainClass object and its attached MainInfo object to a ConditionalWeakTable<TKey, TValue> table. The example also illustrates calls to the Add 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";
      else
         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";
      else
         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


Silverlight

Supported in: 5, 4

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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