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The MSDN documentation states:
Expression.Quote
Method Creates a UnaryExpression that represents an expression that has a constant value of type Expression.
I've been able to build predicate expressions for use in LINQ queries by manually constructing them using the Expression class, but have never come across the need for Expression.Quote.
When and why would you use this? From the LINQ expressions I've seen that have them, they just seem to wrap existing expressions without adding any value.
What is the purpose of the Quote method/node type?
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Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language. Traditionally, queries against data are expressed as simple strings without type checking at compile time or IntelliSense support.
Object. Gets the Expression that represents the instance for instance method calls or null for static method calls. Type. Gets the static type of the expression that this Expression represents.
NET, Expression is an abstract class which contains static methods and inherited by various types (like ParameterExpression, MethodCallExpression, BinaryExpression etc.) to create expression tree nodes of specific types. Also all these expression-specific expression tree types are defined in System. Linq.
LINQ introduced the new type called Expression that represents strongly typed lambda expression. It means lambda expression can also be assigned to Expression<TDelegate> type. The . NET compiler converts the lambda expression which is assigned to Expression<TDelegate> into an Expression tree instead of executable code.
Expression.Quote specifies that a lambda is to be treated as an expression tree and not as a function. It induces closure semantics on its operand.
When you are constructing a MethodCallExpression using Expression.Call, any parameters that are lambda expressions (LambdaExpression/Expression<TDelegate>) must use Expression.Quote to wrap the parameter before passing in.
So for a parameter of type Expression<Func<bool>>, when you create an instance such as: () => true, the expression's Type property would be Func<bool> whereas the expression's type (calling GetType) would be Expression<Func<bool>>
So to get an Expression that has the correct value for the Type property you pass the lambda expression into Expression.Quote and pass that as the parameter to Expression.Call.
I had a look at Expression.Quote through reflector and while the sole parameter is of type Expression, it must derive from LambdaExpression and this is checked inside the method. Out of interest, anyone know why MS didn't just make the parameter type be LambdaExpression?
As StevenH pointed out, Expression.Quote is used in implementing LINQ Query Providers. All the methods on Queryable that take a lambda expression such as Where, OrderBy, GroupBy, etc internally construct a MethodCallExpression using Expression.Call and wrap the lambda expression parameters with Expression.Quote calls.
For a more detailed explanation of Expression.Quote read this answer.
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