!= null vs. =null

Question

See below code first please.

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class Program
    {
        public struct MyStruct
        {
            public List<MyStructItem> Items;
        }
        public struct MyStructItem
        {
            public string Value;
        }


        static void Main(string[] args)
        {
            List<MyStruct> myList = new List<MyStruct>();
            myList.Add(new MyStruct());

            //(!) it haven't comipled.
            if (myList[0].Items = null){Console.WriteLine("null!");}

            //(!) but it have compiled.
            if (myList[0].Items != null) { Console.WriteLine("not null!"); }

        }
    }
}

What is difference between !=null and =null in that case?

thanks.

Adam Houldsworth · Accepted Answer · 2011-07-28 08:30:10Z

15

You are using the assignment operator = instead of the equality operator ==.

Try:

//(!) it haven't comipled.            
if (myList[0].Items == null){Console.WriteLine("null!");}            

//(!) but it have compiled.            
if (myList[0].Items != null) { Console.WriteLine("not null!"); }

The difference is one compiles and one doesn't :-)

C# Operators:

http://msdn.microsoft.com/en-us/library/6a71f45d(v=vs.80).aspx

edited Jul 28, 2011 at 8:30

answered Jul 28, 2011 at 8:09

Adam Houldsworth

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3 Comments

AAT Over a year ago

Why does the compiler not compile 'myList[0].Items = null'? Is myList readonly?

AAT Over a year ago

Having thought about it for another minute or so, I guess the answer to my own question is that the problem is that the C# compiler won't automatically convert an object to a bool. As an old C/C++ hand I keep forgetting how different C# is!

Adam Houldsworth Over a year ago

@AAT it will complain that it can't convert it to a bool. myList[0].Items = null on its own will compile.

Sergey Metlov · Accepted Answer · 2011-07-28 08:09:56Z

5

= null is assignment. You should use == null

You assign the null value to myList[0].Items and tries to use it as bool in if statement. That is why code can not be compiled.
For example, this code compiles successfully:

bool b;
if (b = true)
{
    ...
}

Because you set true value to b and then check it in if statement.

answered Jul 28, 2011 at 8:09

Sergey Metlov

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Comments

Tjekkles · Accepted Answer · 2011-07-28 08:09:37Z

3

=null you set the value to null

!= null you check if it is different from null

If you want to compare if it is equal to null, use == null instead of = null.

answered Jul 28, 2011 at 8:09

Tjekkles

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Comments

Donatas K. · Accepted Answer · 2011-07-28 08:10:25Z

3

'=' is for assignment, not for comparing. Use '=='

answered Jul 28, 2011 at 8:10

Donatas K.

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Comments

Soony · Accepted Answer · 2011-07-28 08:09:47Z

2

if (myList[0].Items == null){Console.WriteLine("null!");}

answered Jul 28, 2011 at 8:09

Soony

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Comments

Øyvind Bråthen · Accepted Answer · 2011-07-28 08:10:45Z

2

First off, myList[0].Items = null will set the object to null. You probably mean myList[0].Items == null

And regarding the difference, == checked if something is equal. != checks if something is not equal.

answered Jul 28, 2011 at 8:10

Øyvind Bråthen

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Comments

Kirill Polishchuk · Accepted Answer · 2011-07-28 08:10:40Z

1

For predefined value types, the equality operator (==) returns true if the values of its operands are equal, false otherwise. For reference types other than string, == returns true if its two operands refer to the same object. For the string type, == compares the values of the strings.

And

The assignment 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. The operands must be of the same type (or the right-hand operand must be implicitly convertible to the type of the left-hand operand).

Reference: http://msdn.microsoft.com/en-us/library/6a71f45d(v=vs.71).aspx

answered Jul 28, 2011 at 8:10

Kirill Polishchuk

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Jon Egerton · Accepted Answer · 2011-07-28 08:10:57Z

1

= is the assignment operator, you should use the equality operator ==

answered Jul 28, 2011 at 8:10

Jon Egerton

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PVitt · Accepted Answer · 2011-07-28 08:11:06Z

1

= null is an assignment. == null is a condition.

answered Jul 28, 2011 at 8:11

PVitt

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foxy · Accepted Answer · 2011-07-28 08:12:10Z

1

if (myList[0].Items != null)

is a negative comparison. It checks if myList[0].Items is not equal to null.

if (myList[0].Items = null)

is an assignment. It would normally assign null to myList[0].Items and return true (in languages like C++), however, in C#, this won't compile (because of this common error).

answered Jul 28, 2011 at 8:12

foxy

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