Статья
11/18/2011

Expression.Switch Method (Expression, Expression, array<SwitchCase[])

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Creates a SwitchExpression that represents a switch statement that has a default case.

Namespace: System.Linq.Expressions
Assembly: System.Core (in System.Core.dll)

Syntax

'Declaration
Public Shared Function Switch ( _
    switchValue As Expression, _
    defaultBody As Expression, _
    ParamArray cases As SwitchCase() _
) As SwitchExpression

public static SwitchExpression Switch(
    Expression switchValue,
    Expression defaultBody,
    params SwitchCase[] cases
)

Parameters

switchValue
Type: System.Linq.Expressions.Expression
The value to be tested against each case.

defaultBody
Type: System.Linq.Expressions.Expression
The result of the switch if switchValue does not match any of the cases.

cases
Type: array<System.Linq.Expressions.SwitchCase[]
The set of cases for this switch expression.

Return Value

Type: System.Linq.Expressions.SwitchExpression
The created SwitchExpression.

Remarks

All SwitchCase objects in a SwitchExpression object must have the same type, unless the SwitchExpression has the type void.

Each SwitchCase object has an implicit break statement, which means that there is no implicit fall through from one case label to another.

If switchValue does not match any of the cases, the default case represented by defaultBody is run.

Examples

The following example demonstrates how to create an expression that represents a swtich statement that has a default case.

' Add the following directive to the file:
' Imports System.Linq.Expressions

' An expression that represents the switch value.
Dim switchValue As ConstantExpression = Expression.Constant(3)

' This expression represents a switch statement 
' that has a default case.
Dim switchExpr As SwitchExpression =
Expression.Switch(
    switchValue,
    Expression.Call(
                Nothing,
                GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
                Expression.Constant("Default")
            ),
    New SwitchCase() {
        Expression.SwitchCase(
            Expression.Call(
                Nothing,
                GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
                Expression.Constant("First")
            ),
            Expression.Constant(1)
        ),
        Expression.SwitchCase(
            Expression.Call(
                Nothing,
                GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
                Expression.Constant("Second")
            ),
            Expression.Constant(2)
        )
    }
)

' The following statement first creates an expression tree,
' then compiles it, and then runs it.
Expression.Lambda(Of Action)(switchExpr).Compile()()

' This code example produces the following output:
'
' Default

// Add the following directive to the file:
// using System.Linq.Expressions;  

// An expression that represents the switch value.
ConstantExpression switchValue = Expression.Constant(3);

// This expression represents a switch statement 
// that has a default case.
SwitchExpression switchExpr =
    Expression.Switch(
        switchValue,
        Expression.Call(
                    null,
                    typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
                    Expression.Constant("Default")
                ),
        new SwitchCase[] {
               Expression.SwitchCase(
                   Expression.Call(
                       null,
                       typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
                       Expression.Constant("First")
                   ),
                   Expression.Constant(1)
               ),
               Expression.SwitchCase(
                   Expression.Call(
                       null,
                       typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
                       Expression.Constant("Second")
                   ),
                   Expression.Constant(2)
               )
           }
    );

// The following statement first creates an expression tree,
// then compiles it, and then runs it.
Expression.Lambda<Action>(switchExpr).Compile()();

// This code example produces the following output:
//
// Default

Version Information

Silverlight

Supported in: 5, 4

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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