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Learn more about: How to: Execute Expression Trees (Visual Basic)
How to: Execute Expression Trees
07/20/2015
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How to: Execute Expression Trees (Visual Basic)

This topic shows you how to execute an expression tree. Executing an expression tree may return a value, or it may just perform an action such as calling a method.

Only expression trees that represent lambda expressions can be executed. Expression trees that represent lambda expressions are of type xref:System.Linq.Expressions.LambdaExpression or xref:System.Linq.Expressions.Expression%601. To execute these expression trees, call the xref:System.Linq.Expressions.LambdaExpression.Compile%2A method to create an executable delegate, and then invoke the delegate.

Note

If the type of the delegate is not known, that is, the lambda expression is of type xref:System.Linq.Expressions.LambdaExpression and not xref:System.Linq.Expressions.Expression%601, you must call the xref:System.Delegate.DynamicInvoke%2A method on the delegate instead of invoking it directly.

If an expression tree does not represent a lambda expression, you can create a new lambda expression that has the original expression tree as its body, by calling the xref:System.Linq.Expressions.Expression.Lambda%60%601%28System.Linq.Expressions.Expression%2CSystem.Collections.Generic.IEnumerable%7BSystem.Linq.Expressions.ParameterExpression%7D%29 method. Then, you can execute the lambda expression as described earlier in this section.

Example

The following code example demonstrates how to execute an expression tree that represents raising a number to a power by creating a lambda expression and executing it. The result, which represents the number raised to the power, is displayed.

' The expression tree to execute.  
Dim be As BinaryExpression = Expression.Power(Expression.Constant(2.0R), Expression.Constant(3.0R))  
  
' Create a lambda expression.  
Dim le As Expression(Of Func(Of Double)) = Expression.Lambda(Of Func(Of Double))(be)  
  
' Compile the lambda expression.  
Dim compiledExpression As Func(Of Double) = le.Compile()  
  
' Execute the lambda expression.  
Dim result As Double = compiledExpression()  
  
' Display the result.  
MsgBox(result)  
  
' This code produces the following output:  
' 8

Compile the code

  • Include the System.Linq.Expressions namespace.

See also