Will using 'var' affect performance?

There's no extra Intermediate language (IL) code for the var keyword: the resulting IL should be identical for non-anonymous types. If the compiler can't create that IL because it can't figure out what type you intended to use, you'll get a compiler error.

The only trick is that var will infer an exact type where you may have chosen an Interface or parent type if you were to set the type manually.

As Joel says, the compiler works out at compile-time what type var should be, effectively it's just a trick the compiler performs to save keystrokes, so for example

var s = "hi";

gets replaced by

string s = "hi";

by the compiler before any IL is generated. The Generated IL will be exactly the same as if you'd typed string.

The C# compiler infers the true type of the var variable at compile time. There's no difference in the generated IL.

I don't think you properly understood what you read. If it gets compiled to the correct type, then there is no difference. When I do this:

var i = 42;

The compiler knows it's an int, and generate code as if I had written

int i = 42;

As the post you linked to says, it gets compiled to the same type. It's not a runtime check or anything else requiring extra code. The compiler just figures out what the type must be, and uses that.

There is no runtime performance cost to using var. Though, I would suspect there to be a compiling performance cost as the compiler needs to infer the type, though this will most likely be negligable.

If the compiler can do automatic type inferencing, then there wont be any issue with performance. Both of these will generate same code

var    x = new ClassA();
ClassA x = new ClassA();

however, if you are constructing the type dynamically (LINQ ...) then var is your only question and there is other mechanism to compare to in order to say what is the penalty.

It's depends of situation, if you try use, this code bellow.

The expression is converted to "OBJECT" and decrease so much the performance, but it's a isolated problem.

CODE:

public class Fruta
{
    dynamic _instance;

    public Fruta(dynamic obj)
    {
        _instance = obj;
    }

    public dynamic GetInstance()
    {
        return _instance;
    }
}

public class Manga
{
    public int MyProperty { get; set; }
    public int MyProperty1 { get; set; }
    public int MyProperty2 { get; set; }
    public int MyProperty3 { get; set; }
}

public class Pera
{
    public int MyProperty { get; set; }
    public int MyProperty1 { get; set; }
    public int MyProperty2 { get; set; }
}

public class Executa
{
    public string Exec(int count, int value)
    {
        int x = 0;
        Random random = new Random();
        Stopwatch time = new Stopwatch();
        time.Start();

        while (x < count)
        {
            if (value == 0)
            {
                var obj = new Pera();
            }
            else if (value == 1)
            {
                Pera obj = new Pera();
            }
            else if (value == 2)
            {
                var obj = new Banana();
            }
            else if (value == 3)
            {
                var obj = (0 == random.Next(0, 1) ? new Fruta(new Manga()).GetInstance() : new Fruta(new Pera()).GetInstance());
            }
            else
            {
                Banana obj = new Banana();
            }

            x++;
        }

        time.Stop();
        return time.Elapsed.ToString();
    }

    public void ExecManga()
    {
        var obj = new Fruta(new Manga()).GetInstance();
        Manga obj2 = obj;
    }

    public void ExecPera()
    {
        var obj = new Fruta(new Pera()).GetInstance();
        Pera obj2 = obj;
    }
}

Above results with ILSPY.

public string Exec(int count, int value)
{
    int x = 0;
    Random random = new Random();
    Stopwatch time = new Stopwatch();
    time.Start();

    for (; x < count; x++)
    {
        switch (value)
        {
            case 0:
                {
                    Pera obj5 = new Pera();
                    break;
                }
            case 1:
                {
                    Pera obj4 = new Pera();
                    break;
                }
            case 2:
                {
                    Banana obj3 = default(Banana);
                    break;
                }
            case 3:
                {
                    object obj2 = (random.Next(0, 1) == 0) ? new Fruta(new Manga()).GetInstance() : new Fruta(new Pera()).GetInstance();
                    break;
                }
            default:
                {
                    Banana obj = default(Banana);
                    break;
                }
        }
    }
time.Stop();
return time.Elapsed.ToString();
}

If you wish execute this code use the code bellow, and get the difference of times.

        static void Main(string[] args)
    {
        Executa exec = new Executa();            
        int x = 0;
        int times = 4;
        int count = 100000000;
        int[] intanceType = new int[4] { 0, 1, 2, 3 };

        while(x < times)
        {                
            Parallel.For(0, intanceType.Length, (i) => {
                Console.WriteLine($"Tentativa:{x} Tipo de Instancia: {intanceType[i]} Tempo Execução: {exec.Exec(count, intanceType[i])}");
            });
            x++;
        }

        Console.ReadLine();
    }

Regards