What is the difference between Expression.Variable() and Expression.Parameter()?

Question

Both seem to return the same type, and have the same signature.

So what is the difference between them, and when should we use each?

Answer 1

Effectively, there is no difference, apart from the fact that Variable() does not allow ref types. To see that, you can look at the reference source:

public static ParameterExpression Parameter(Type type, string name) {
    ContractUtils.RequiresNotNull(type, "type");

    if (type == typeof(void)) {
        throw Error.ArgumentCannotBeOfTypeVoid();
    }

    bool byref = type.IsByRef;
    if (byref) {
        type = type.GetElementType();
    }

    return ParameterExpression.Make(type, name, byref);
}

public static ParameterExpression Variable(Type type, string name) {
    ContractUtils.RequiresNotNull(type, "type");
    if (type == typeof(void)) throw Error.ArgumentCannotBeOfTypeVoid();
    if (type.IsByRef) throw Error.TypeMustNotBeByRef();
    return ParameterExpression.Make(type, name, false);
}

As you can see, both methods call ParameterExpression.Make(), so the returned object will behave the same.