What is the purpose of LINQ's Expression.Quote method?

Question

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?

Answer (Helped by StevenH)

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.

Answer 1

It is used to create a UnaryExpression of any node type that represents a unary operation.

In mathematics, a unary operation is an operation with only one operand, i.e. a single input. Specifically, it is a function.

In the C family of languages, the following operators are unary:

• Increment: ++x, x++
• Decrement: −−x, x−−
• Address: &x
• Indirection: *x
• Positive: +x
• Negative: −x
• One's complement: ~x
• Logical negation: !x

Let's look at the following C# code:

//Create a function which is a "Unary Operation" 
//- Has only 1 operand
//- Has a single input
//- Is constant
Func<int, int> inc = x => x + 1; // x being the single operand, and +1 (++) being the operation)

//Convert the function into an Expression
Expression<Func<int,int>> exp = y => inc(y);

//Convert the expression into a "Unary Expression"
UnaryExpression uexp = Expression.Quote(exp);

In essence the UnaryExpression is a specialised Class for the storage and manipulation of a Unary Operation as defined. You would leverage Unary Expression when you wish to work with the underlying unary operation directly and in a more specialised fashion.

Source for info and definition of Unary Operation (Wikipedia)

Unary Expression gives you extra Members (full list here on MSDN) e.g. Access to the operand in the Expression:

//Convert the expression into a "Unary Expression"
UnaryExpression uexp = Expression.Quote(exp);

Expression op = uexp.Operand; //The operand being constant

In the screenshot below you can see that the:

y => inc(y);

...is NodeType Constant and the definition of the:

UnaryExpression uexp = Expression.Quote(exp);

...is "Expression.Quote Method Creates a UnaryExpression that represents an expression that has a constant value of type Expression."

Update: Looking at specific Unary Expression NodeTypes

Lets simplify this example further to drive out some meaning:

Expression<Func<int, int>> exp = x => x + 1;

//Convert the expression into a "Unary Expression"
UnaryExpression uexp = Expression.Quote(exp);

This time the expression 'exp' will have the NodeType Add as shown below:

Expression<Func<int, int>> exp = x => x - 1;

//Convert the expression into a "Unary Expression"
UnaryExpression uexp = Expression.Quote(exp);

This time the expression 'exp' will have the NodeType Substract as shown below:

Add & Subtract are Unary operations. See the full list which was quoted at the beginning of this answer ("In the C family of languages, the following operators are unary: ..").

If you look at the UnaryExpression objects NodeType it is Quote:

This, as I mentioned briefly before, is why you would use Expression.Quote, to store and gain access to information regarding the UnaryExpression, that is it's purpose.

The most common usage I have found in real world development is within the implementation of Linq providers when expression are passed in as paramters for conversion & evaluation against some source that implements IQueryable.

The paramter in Expression.Where(..) takes a predicate and when that predicate is being executed by the custom provider it needs to be "Quoted" and passed to the providers executor as an argument of type Expression. See:"Understanding LINQ to SQL - Implementing LINQ to SQL Provider" for implementation code.