Question

I am trying to use logical operators in the following lambda expression

int count = dataContext.Users.Count(u => u.ID == 1 && u.Name == "name");

below is the complete function

private void Login(int id, string password)
{
    MyDataContext dataContext = new MyDataContext();
    int count = dataContext.Users.Count(u => u.ID == 1 && u.Name == "name");
}

I’m using Microsoft SQL Server 2008 Express 2008 Edition and User table looks like

ID int autogenerated
Name string

but Visual Studio 2010 gives the following design time errors “Invalid expression terms”. Any idea what I might be doing wrong?

I get the following error messages

• Error 2 ) expected …\FormAdd.cs 105 123
• Error 3 ; expected …\FormAdd.cs 105 126
• Error 5 ; expected …\FormAdd.cs 105 158
• Error 7 ; expected …\FormAdd.cs 105 202
• Error 4 Invalid expression term ‘)’ …\FormAdd.cs 105 158
• Error 6 Invalid expression term ‘)’ …\FormAdd.cs 105 202
• Error 1 Invalid expression term ‘=>’ …\FormAdd.cs 105 123

Answer 1

The code you show in the screenshot is

int count = dataContext.Users.Count(u => u.ID == 1 && u => u.Name == "name");
                                                        **

This is illegal code, and the part with the ** underneath it is the source of the error.

The code you show in the body of your question is

int count = dataContext.Users.Count(u => u.ID == 1 && u.Name == "name");

These are different. The first, is incorrect, as the IDE points out to you. The latter, is correct.

The signature for the overload of Count you are using is

 int Count<T>(this IEnumerable<T> source, Predicate<T> predicate);

This means to invoke it on dataContext.Users, you need to provide a Predicate<User>. A Predicate<User> is a function that maps User to bool. One way to write this is

 static bool UserPredicate(User u) {
     return u.ID == 1 && u.Name == "name";
 }

and then you can say

int count = dataContext.Users.Count(UserPredicate);

Another way is to say

int count = dataContext.Users.Count(
    delegate(User u) { return u.ID == 1 && u.Name == "name" }
);

This lets you define the predicate inline. It’s a nice feature. Note that the expression that effectively defines the predicate

u.ID == 1 && u.Name == "name"

is the same as in the case of UserPredicate defined above.

And finally, Lambda expressions allow you to get rid of some of that unnecessary fluff.

int count = dataContext.Users.Count(
    u => f(u)
)

Where f(u) is an expression in u. One expression that you can use is

u.ID == 1 && u.Name == "name"

Again, the same expression. See, your attempt in the IDE was illegal because you tried to write

u.ID == 1 && u => u.Name == "name"

and that is clearly not a legal expression.