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<(Of <(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<(Of <(TDelegate>)>)(Expression, IEnumerable<(Of <(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
See Also