Why C# Need Integral Types already it has "Var" keyword? [duplicate]

This question already has answers here: Use of var keyword in C# (86 answers) Closed 8 years ago.

That can be silly question but I really don't know the reason.

    // i is compiled as an int
    var i = 5;

    // s is compiled as a string
    var s = "Hello";

    // a is compiled as int[]
    var a = new[] { 0, 1, 2 };

    // expr is compiled as IEnumerable<Customer>
    // or perhaps IQueryable<Customer>
    var expr =
        from c in customers
        where c.City == "London"
        select c;

    // anon is compiled as an anonymous type
    var anon = new { Name = "Terry", Age = 34 };

    // list is compiled as List<int>                             
    var list = new List<int>();

If so, why C# needs integral types like int, string, int[], List&l开发者_JAVA技巧t;> ? If we can do that only with var keywords, why are there integral types?

Another way,

What is the difference between these ?

var s = "Hello";
string s = "Hello";

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in no particular order...

• var is only a compiler construct; even var i = 4; is typed as int
• C# hasn't always had var, and you can't remove keywords without utterly breaking existing code.
• you can only use var in method variables - not fields, parameters, return types etc.
• most people are happy to type int in place of var! Especially when the type matters and there are bytes, floats, enums, decimals etc that all work with the literal 0; i.e. short x = 0; int y = 0; long z = 0; - why change only int to var there?
• you can't use var to specify a generic type parameter (<T>) or in a typeof
• you can't use var.TryParse in place of int.TryParse

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Many people would argue that it is better practice to write int i = 5 rather than var i = 5. There are many reasons for this, but the most compelling to me is that I can look at the code and know immediately, without stretching my brain, what type i is. This allows my brain to concentrate on the harder problems.

In fact var was introduced to declare variables whose types could not easily be named. It was not introduced as a convenience, although it can be used that way.

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The var keyword is just syntactic sugar, which can only be used for local variables in methods. It's still a strongly typed variable. You can hover the mouse over it in visual studio to see what type it is.

We still need types as otherwise how would you do var list = new List<int>();? You need to type List<int> somewhere!

In many cases its best to be explicit, for example when working with literals that could be a number of different types, or to increase readability.

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There is no difference between var s = "Hello";" and string s ="Hello";". However, there's a big difference between var foo = new Foo(); and IFoo foo = new Foo();. In the first case all of Foo's members are accessible, in the latter it's ensured that only interface members is used. This may be important to ensure that the behavior is compatible with the interface, not only the concrete implementations.

Also, var can't be used to pass function arguments or to declare member variables. As of to use or not to use var, I refer to this question.