How to: Execute Expression Trees (C# and 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 LambdaExpression or Expression<TDelegate>. To execute these expression trees, call the Compile method to create an executable delegate, and then invoke the delegate.

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 Lambda<TDelegate>(Expression, IEnumerable<ParameterExpression>) 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

// The expression tree to execute.
            BinaryExpression be = Expression.Power(Expression.Constant(2D), Expression.Constant(3D));

            // Create a lambda expression.
            Expression<Func<double>> le = Expression.Lambda<Func<double>>(be);

            // Compile the lambda expression.
            Func<double> compiledExpression = le.Compile();

            // Execute the lambda expression. 
            double result = compiledExpression();

            // Display the result.
            Console.WriteLine(result);

            // This code produces the following output: 
            // 8

Compiling the Code

• Add a project reference to System.Core.dll if it is not already referenced.

• Include the System.Linq.Expressions namespace.

See Also

Tasks

How to: Modify Expression Trees (C# and Visual Basic)

Concepts

Expression Trees (C# and Visual Basic)