to compile and run an expression tree. Invoking the delegate and writing the result to the console. Creating an expression tree.Įxpression> expr = num => num result = expr.Compile()
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The following code example demonstrates how to compile an expression tree and run the resulting code. The Expression type provides the Compile method that compiles the code represented by an expression tree into an executable delegate. For more information, see How to modify expression trees (C#). You can use an expression tree visitor to traverse the existing expression tree. This means that if you want to modify an expression tree, you must construct a new expression tree by copying the existing one and replacing nodes in it. Decomposed expression: num => num LessThan 5Įxpression trees should be immutable. This code produces the following output: Param.Name, left.Name, operation.NodeType, right.Value) The following code example demonstrates how the expression tree that represents the lambda expression num => num > exprTree = num => num ", Int factorial = Expression.Lambda>(block, value).Compile()(5) įor more information, see Generating Dynamic Methods with Expression Trees in Visual Studio 2010, which also applies to later versions of Visual Studio. Compile and execute an expression tree. If false, exit the loop and go to the label. Adding a conditional block into the loop.Įxpression.GreaterThan(value, Expression.Constant(1)), Assigning a constant to a local variable: result = 1Įxpression.Assign(result, Expression.Constant(1)), LabelTarget label = Expression.Label(typeof(int)) īlockExpression block = Expression.Block( Creating a label to jump to from a loop. ParameterExpression result = Expression.Parameter(typeof(int), "result") Creating an expression to hold a local variable. ParameterExpression value = Expression.Parameter(typeof(int), "value") The following example demonstrates how to create an expression tree that calculates the factorial of a number. By using the API, you can create expression trees that are more complex than those that can be created from lambda expressions by the C# compiler. NET Framework 4 or later, the expression trees API also supports assignments and control flow expressions such as loops, conditional blocks, and try-catch blocks. The following code examples demonstrate how to have the C# compiler create an expression tree that represents the lambda expression num => num > lambda = num => num num num > lambda1 = For more information about lambda expressions in C#, see Lambda Expressions. It cannot parse statement lambdas (or multi-line lambdas).
The C# compiler can generate expression trees only from expression lambdas (or single-line lambdas). Expression trees represent code in a tree-like data structure, where each node is an expression, for example, a method call or a binary operation such as x, the compiler emits code to build an expression tree that represents the lambda expression.