System.Linq.Expressions Namespace

The System.Linq.Expressions namespace contains classes, interfaces and enumerations that enable language-level code expressions to be represented as objects in the form of expression trees.



Represents an expression that has a binary operator.


Represents a block that contains a sequence of expressions where variables can be defined.


Represents a catch statement in a try block.


Represents an expression that has a conditional operator.


Represents an expression that has a constant value.


Emits or clears a sequence point for debug information. This allows the debugger to highlight the correct source code when debugging.


Represents the default value of a type or an empty expression.


Represents a dynamic operation.


Represents a visitor or rewriter for dynamic expression trees.


Represents an initializer for a single element of an IEnumerable collection.


Provides the base class from which the classes that represent expression tree nodes are derived. It also contains static (Shared in Visual Basic) factory methods to create the various node types. This is an abstract class.


Represents a strongly typed lambda expression as a data structure in the form of an expression tree. This class cannot be inherited.


Represents a visitor or rewriter for expression trees.


Represents an unconditional jump. This includes return statements, break and continue statements, and other jumps.


Represents indexing a property or array.


Represents an expression that applies a delegate or lambda expression to a list of argument expressions.


Represents a label, which can be put in any Expression context. If it is jumped to, it will get the value provided by the corresponding GotoExpression. Otherwise, it receives the value in DefaultValue. If the Type equals System.Void, no value should be provided.


Used to represent the target of a GotoExpression.


Describes a lambda expression. This captures a block of code that is similar to a .NET method body.


Represents a constructor call that has a collection initializer.


Represents an infinite loop. It can be exited with "break".


Represents assignment operation for a field or property of an object.


Provides the base class from which the classes that represent bindings that are used to initialize members of a newly created object derive.


Represents accessing a field or property.


Represents calling a constructor and initializing one or more members of the new object.


Represents initializing the elements of a collection member of a newly created object.


Represents initializing members of a member of a newly created object.


Represents a call to either static or an instance method.


Represents creating a new array and possibly initializing the elements of the new array.


Represents a constructor call.


Represents a named parameter expression.


An expression that provides runtime read/write permission for variables.


Represents one case of a SwitchExpression.


Represents a control expression that handles multiple selections by passing control to SwitchCase.


Stores information necessary to emit debugging symbol information for a source file, in particular the file name and unique language identifier.


Represents a try/catch/finally/fault block.


Represents an operation between an expression and a type.


Represents an expression that has a unary operator.



Provides an internal interface for accessing the arguments of multiple tree nodes (DynamicExpression, ElementInit, MethodCallExpression, InvocationExpression, NewExpression, and IndexExpression). You should not use this API. It is only public due to DLL refactoring and exists only for internal performance optimizations.

This enables two optimizations which reduce the size of expression trees. The first enables the tree nodes to hold onto an IList<T> instead of a ReadOnlyCollection. IList<T> saves the cost of allocating the ReadOnlyCollection for each node. The second enables creating specialized subclasses that hold onto a specific number of arguments (for example, Block2, Block3, Block4). Therefore, these nodes avoid allocating both a ReadOnlyCollection and an array for storing their elements, saving 32 bytes per node.

The expression tree nodes continue to expose the original LINQ properties of ReadOnlyCollections. The nodes do this by re-using a field for storing both the array or an element that would normally be stored in the array.

For the array case, the collection is typed to IList<T> instead of ReadOnlyCollection<T>. When the node is initially constructed it is an array. When the compiler accesses the members it uses this interface. Accessing array elements promotes the array to a ReadOnlyCollection.

For the object case we store the first argument in a field typed to object. When the node is initially constructed, the field holds the Expression. The compiler accesses arguments through this interface, and the accessor for the first argument uses Expression.ReturnObject to return the object that handles the Expression or ReadOnlyCollection case. When the user accesses the ReadOnlyCollection, then the object field is updated to hold directly onto the ReadOnlyCollection.

It is important that the Expressions consistently return the same ReadOnlyCollection. Otherwise, the re-writer tree walker will break. It is a breaking change from LINQ v1 to return different ReadOnlyCollections form the same Expression node. Currently users can rely on object identity to tell if the node has changed. Storing the ReadOnlyCollection in an overloaded field both reduces memory usage and maintains compatibility for the public API.


Provides an internal interface for accessing the arguments of DynamicExpression tree nodes as well as CallSite and Rewriting functionality. You should not use this API. It is only public due to DLL refactoring and exists only for internal performance optimizations.



Describes the node types for the nodes of an expression tree.


Specifies what kind of jump this GotoExpression represents.


Describes the binding types that are used in MemberInitExpression objects.


The abstract class Expression provides the root of a class hierarchy used to model expression trees.

The classes in this namespace that derive from Expression, for example MemberExpression and ParameterExpression, are used to represent nodes in an expression tree. The Expression class contains static (Shared in Visual Basic) factory methods to create expression tree nodes of the various types.

The enumeration type ExpressionType specifies the unique node types.