ConditionalWeakTable<TKey,TValue>.GetValue Method

Definition

Namespace:

System.Runtime.CompilerServices

Assemblies:

netstandard.dll, System.Runtime.dll

Source:

ConditionalWeakTable.cs

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 a callback method to create a value that is bound to the specified key.

C#

public TValue GetValue(TKey key, System.Runtime.CompilerServices.ConditionalWeakTable<TKey,TValue>.CreateValueCallback createValueCallback);

Parameters

key

TKey

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

createValueCallback

ConditionalWeakTable<TKey,TValue>.CreateValueCallback

A delegate to a method that can create a value for the given key. It has a single parameter of type TKey, and returns a value of type TValue.

Returns

TValue

The value attached to key, if key already exists in the table; otherwise, the new value returned by the createValueCallback delegate.

Exceptions

ArgumentNullException

key or createValueCallback is null.

Examples

The following example defines a MainClass class and a MainInfo class, which provides information about the MainClass instance. It also defines a static (Shared in Visual Basic) CreateAttachedValue method that can be assigned to the ConditionalWeakTable<TKey,TValue>.CreateValueCallback delegate and passed to the GetValue method. The example calls the GetValue 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 GetOrCreateValue methods to add key/value pairs to the table, and to the TryGetValue method to retrieve the value of an existing key.

C#

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);
      else
         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);
      else 
         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

Remarks

If key does not exist in the table, GetValue invokes the method that is defined by the createValueCallback parameter and passes it the key. A new value is bound to the key in the table and returned as a result.

Use this method only when the class that represents the table's value does not define a parameterless constructor. If it does define a parameterless constructor, use the GetOrCreateValue method instead. 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.

If multiple threads try to create the same key, createValueCallback may be invoked multiple times with the same key. Only one of these calls will succeed, and its returned value will be added to the table. Which thread succeeds in creating the value is indeterminate. This rule permits the table to invoke createValueCallback outside the internal table lock to prevent deadlocks.

Applies to

See also

• GetOrCreateValue(TKey)
• TryGetValue(TKey, TValue)
• ConditionalWeakTable<TKey,TValue>.CreateValueCallback