usage of var keyword in C#

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This statement is from Remark section of 'var' from MSDN:

"If a type named var is in scope, then the var keyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration."

I am not getting clear understanding on this remark. Could anyone elaborate it?

https://msdn.microsoft.com/en-us/library/bb384061.aspx[^]

Posted 7-Apr-15 19:17pm

utkarsharaut

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Sergey Alexandrovich Kryukov 8-Apr-15 2:11am

I agree, this phrase is not tremendously clear, so I up-voted this question; it is useful. You got obviously correct answers though.
—SA

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Sascha Lefèvre · Accepted Answer · 2015-04-07T19:41:00

var normally is a C#-Keyword that is used to declare a variable of which the compiler can infer the type by the assignment on the right side:

C#

var i = 1;            // i is of type Int32
var s = "abc";        // s is of type String
var d = DateTime.Now; // d is of type DateTime

var z; // not allowed because the compiler can't infer any type

But you are allowed to define an own type (a class or struct) with the name "var":

C#

class var
{
   // ...
}

var x; // x is of type var

"If a type named var is in scope" - means that the example above would work wherever your declaration of a type named var is known at. If you would declare the type var in a different namespace and not specify that namespace with a using-namespace-directive, then the compiler wouldn't know about that type and again treat an occurence of "var" as the C#-keyword.

This is -or at least should be- of academic interest only, it would be very bad practice to actually define a usertype named var because it will irritate everyone looking at the code.

Serge Desmedt · Accepted Answer · 2015-04-07T19:46:00

paste the following in a visual studio console project and compile it:

C#

namespace ATypeNamedVar
{
    class JustCreatingAScope
    {
        class var
        {
            public string SomeProperty { get; set; }
        }

        public static void AMethodUsingTheVarClass()
        {
            //var thisWillNotCompile = 1;
            var thisWillCompile = new var();
            Console.WriteLine("The variable thisWillCompile is of type: " + thisWillCompile.GetType().ToString());
        }

    }

    class Program
    {
        static void Main(string[] args)
        {
            var regularUse = 1;
            Console.WriteLine("The variable thisWillCompile is of type: " + regularUse.GetType().ToString());

            JustCreatingAScope.AMethodUsingTheVarClass();
        }
    }
}

Note the syntax highlighting in visual studio:
In the first use you get the type colouring and not the keyword coloring (unlike in the above)

So: to answer your question: if you have a type named var, like in the above, if you use the word var it will be interpreted by the compiler is that type and not as the keyword var. That is why the statement

var thisWillNotCompile = 1

does not compile, because you cannot assign an integer to the type var.

usage of var keyword in C#

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