Articolo
11/18/2011

IDictionary<TKey, TValue>.TryGetValue Method

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Gets the value associated with the specified key.

Namespace: System.Collections.Generic
Assembly: mscorlib (in mscorlib.dll)

Syntax

'Declaration
Function TryGetValue ( _
    key As TKey, _
    <OutAttribute> ByRef value As TValue _
) As Boolean

bool TryGetValue(
    TKey key,
    out TValue value
)

Parameters

key
Type: TKey
The key whose value to get.

value
Type: TValue%
When this method returns, the value associated with the specified key, if the key is found; otherwise, the default value for the type of the value parameter. This parameter is passed uninitialized.

Return Value

Type: System.Boolean
true if the object that implements IDictionary<TKey, TValue> contains an element with the specified key; otherwise, false.

Exceptions

Exception	Condition
ArgumentNullException	key is nulla null reference (Nothing in Visual Basic).

Remarks

This method combines the functionality of the ContainsKey method and the Item property.

If the key is not found, then the value parameter gets the appropriate default value for the value type V; for example, zero (0) for integer types, false for Boolean types, and nulla null reference (Nothing in Visual Basic) for reference types.

Examples

The example shows how to use the TryGetValue method to retrieve values. If a program frequently tries key values that are not in a dictionary, the TryGetValue method can be more efficient than using the Item property (the indexer in C#), which throws exceptions when attempting to retrieve nonexistent keys.

This code is part of a larger example that can be compiled and executed. See System.Collections.Generic.IDictionary<TKey, TValue>.

' When a program often has to try keys that turn out not to
' be in the dictionary, TryGetValue can be a more efficient 
' way to retrieve values.
Dim value As String = ""
If openWith.TryGetValue("tif", value) Then
   outputBlock.Text &= String.Format("For key = ""tif"", value = {0}.", value) & vbCrLf
Else
   outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
End If


...


' The default Item property throws an exception if the requested
' key is not in the dictionary.
Try
   outputBlock.Text &= String.Format("For key = ""tif"", value = {0}.", _
       openWith("tif")) & vbCrLf
Catch
   outputBlock.Text &= "Key = ""tif"" is not found." & vbCrLf
End Try

// When a program often has to try keys that turn out not to
// be in the dictionary, TryGetValue can be a more efficient 
// way to retrieve values.
string value = "";
if (openWith.TryGetValue("tif", out value))
{
   outputBlock.Text += String.Format("For key = \"tif\", value = {0}.", value) + "\n";
}
else
{
   outputBlock.Text += "Key = \"tif\" is not found." + "\n";
}


...


// The indexer throws an exception if the requested key is
// not in the dictionary.
try
{
   outputBlock.Text += String.Format("For key = \"tif\", value = {0}.",
       openWith["tif"]) + "\n";
}
catch (KeyNotFoundException)
{
   outputBlock.Text += "Key = \"tif\" is not found." + "\n";
}

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

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

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