Expression<TDelegate> Class
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Represents a strongly typed lambda expression as a data structure in the form of an expression tree. This class cannot be inherited.
System.Linq.Expressions::Expression
System.Linq.Expressions::LambdaExpression
System.Linq.Expressions::Expression<TDelegate>
Assembly: System.Core (in System.Core.dll)
The Expression<TDelegate> type exposes the following members.
| Name | Description | |
|---|---|---|
 | Body | Gets the body of the lambda expression. (Inherited from LambdaExpression.) |
| CanReduce | Indicates that the node can be reduced to a simpler node. If this returns true, Reduce() can be called to produce the reduced form. (Inherited from Expression.) |
| Name | Gets the name of the lambda expression. (Inherited from LambdaExpression.) |
| NodeType | Returns the node type of this Expression. (Inherited from LambdaExpression.) |
| Parameters | Gets the parameters of the lambda expression. (Inherited from LambdaExpression.) |
| ReturnType | Gets the return type of the lambda expression. (Inherited from LambdaExpression.) |
| TailCall | Gets the value that indicates if the lambda expression will be compiled with the tail call optimization. (Inherited from LambdaExpression.) |
| Type | Gets the static type of the expression that this Expression represents. (Inherited from LambdaExpression.) |
| Name | Description | |
|---|---|---|
 | Accept | Dispatches to the specific visit method for this node type. For example, MethodCallExpression calls the VisitMethodCall. (Inherited from Expression.) |
 | Compile | Compiles the lambda expression described by the expression tree into executable code and produces a delegate that represents the lambda expression. |
| Equals(Object) | Determines whether the specified Object is equal to the current Object. (Inherited from Object.) |
| Finalize | Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.) |
| GetHashCode | Serves as a hash function for a particular type. (Inherited from Object.) |
| GetType | Gets the Type of the current instance. (Inherited from Object.) |
| MemberwiseClone | Creates a shallow copy of the current Object. (Inherited from Object.) |
| Reduce | Reduces this node to a simpler expression. If CanReduce returns true, this should return a valid expression. This method can return another node which itself must be reduced. (Inherited from Expression.) |
| ReduceAndCheck | Reduces this node to a simpler expression. If CanReduce returns true, this should return a valid expression. This method can return another node which itself must be reduced. (Inherited from Expression.) |
| ReduceExtensions | Reduces the expression to a known node type (that is not an Extension node) or just returns the expression if it is already a known type. (Inherited from Expression.) |
| ToString | Returns a textual representation of the Expression. (Inherited from Expression.) |
| Update | Creates a new expression that is like this one, but using the supplied children. If all of the children are the same, it will return this expression. |
| VisitChildren | Reduces the node and then calls the visitor delegate on the reduced expression. The method throws an exception if the node is not reducible. (Inherited from Expression.) |
When a lambda expression is assigned to a variable, field, or parameter whose type is Expression<TDelegate>, the compiler emits instructions to build an expression tree.
 Note: |
|---|
A conversion from a lambda expression to type Expression<D> (Expression(Of D) in Visual Basic) exists if a conversion from the lambda expression to a delegate of type D exists. However, the conversion may fail, for example, if the body of the lambda expression is a block. This means that delegates and expression trees behave similarly with regard to overload resolution. |
The expression tree is an in-memory data representation of the lambda expression. The expression tree makes the structure of the lambda expression transparent and explicit. You can interact with the data in the expression tree just as you can with any other data structure.
The ability to treat expressions as data structures enables APIs to receive user code in a format that can be inspected, transformed, and processed in a custom manner. For example, the LINQ to SQL data access implementation uses this facility to translate expression trees to Transact-SQL statements that can be evaluated by the database.
Many standard query operators defined in the Queryable class have one or more parameters of type Expression<TDelegate>.
The NodeType of an Expression<TDelegate> is Lambda.
Use the Lambda<TDelegate>(Expression, IEnumerable<ParameterExpression>) or Lambda<TDelegate>(Expression, array<ParameterExpression>) method to create an Expression<TDelegate> object.
The following code example demonstrates how to represent a lambda expression both as executable code in the form of a delegate and as data in the form of an expression tree. It also demonstrates how to turn the expression tree back into executable code by using the Compile method.