为什么不能分配LINQ Find（）或FirstOrDefault（）结果的返回值？

Question

class Program
{
    static void Main(string[] args)
    {
        var people = new List<Person>
        {
            new Person { Name = "foo" },
            new Person { Name = "bar" }
        };

        // ERROR: Left-hand side must be a variable, property, or indexer
        people.FirstOrDefault(x => x.Name == "foo") = new Person();
    }
}

public class Person
{
    public string Name { get; set; }
}

Why is the above FirstOrDefault line not valid .NET code? If the return of FirstOrDefault() isn't a variable, what is it?

Edit: I understand how to make this valid .NET code - my point is trying to get a concrete answer as to why this is not valid. A definition from the docs perhaps as to why. I don't need to see how to make it correct :)

Answer 1

you cannot assign to the return of a function you need to store it in a variable

var person = people.FirstOrDefault(x => x.Name == "foo") ?? new Person();

FirstOrDefault is a Get method it is not an array indexer you cannot use it to set values, For instance what do you expect your original statement accomplish

 people.FirstOrDefault(x => x.Name == "foo") = new Person();

you cannot use it for anything afterwards until it is in a variable.

A method returns used for returning values if you want to set in the method you must use an out parameter or reference, or a property.

Answer 2

Why is the above FirstOrDefault line not valid .NET code?

Because you can only assign a value to a variable, and FirstOrDefault doesn't return a variable.

If the return of FirstOrDefault() isn't a variable, what is it?

It's a value. You can store the result of FirstOrDefault in a variable, bit it isn't itself a variable

Answer 3

If you want to assign to the first element of the list, you should just access it at the index: people[0] = new Person() .

To answer your other question, it's not a variable; it's an object. Imagine I tried to write "foo" = "bar" . That would throw an exception -- what does it mean to assign a string literal to another? Your code is logically equivalent.

Answer 4

In C# (currently), you cannot set a value to a function call. Instead, you should say:

var person = people.FirstOrDefault(x => x.Name == "foo");

if (person != null) {
    person = new Person();
}

The C# spec says that the = operator:

stores the value of its right-hand operand in the storage location, property, or indexer denoted by its left-hand operand and returns the value as its result ( https://msdn.microsoft.com/en-us/library/sbkb459w.aspx )

A function call is not a storage location, property, or indexer.

This may sound confusing, but think about it this way:

The = operator is assignment. x = y means that, when you ask for x , give me y (remember basic algebra?).

You cannot say, "for this function, I want you to always give me y " ( FirstOrDefault(x => x.foo) = y ). That kinda goes against a function, which returns a result based on its definition! :)

Answer 5

FirstOrDefault() is a method call and it cannot be used as a variable.

Answer to your question: the value of right hand side is assigned to left hand side, but an object cannot be on the left hand side; only a variable can.

You are trying to assign result of FirstOrDefault() method to People class constructor.

Answer 6

I assume you're trying to retrieve person "foo" to do something with it later. You need to store it in a variable. Your code is not valid C# syntax.

class Program
{
    static void Main(string[] args)
    {
        var people = new List<Person>
        {
            new Person { Name = "foo" },
            new Person { Name = "bar" }
        };

        var fooPerson = people.FirstOrDefault(x => x.Name == "foo");

        // Use fooPerson
    }
}

Answer 7

in the code

people.FirstOrDefault(x => x.Name == "foo") = new Person();

we see an assignment statement. Assigning one value to another. Often with C style languages we call this assigning an lvalue (for left side) an rvalue (or right side.)

lvalue = rvalue.

There are limits on what the lvalue and rvalue can be. The rvalue must be able to be evaluated to the same type as the lvalue and the lvalue must be a location in memory (sometimes called a variable.)

If you want to find out more about these issues you can look at language references and how they talk about the lvalue and rvalue in assignment statements.