C# out vars and discards

Posted Sep 10, 2018

C# 7 introduces an improved syntax for joining declaration and usage of an out variable.

Consider the following example:

int integerVal;
int.TryParse("123", out integerVal);

The same can now be re-written in c# 7 as follows:

int.TryParse("123", out int integerVal);

Notice the int right after the out keyword, which enables you to declare the variable in the method call itself, instead of a separate declaration before the method call. Additionally, you can also use an implicitly typed local variable, instead of defining a typed variable.

int.TryParse("123", out var integerVal);

However a typed declaration is preferable for sake of clarity and readability of code.

Discards These are temporary, dummy variables, which we intentionally want to leave them unused. Consider the following code:

void Main()
{
    Calculate(5, 1, out int sum, out int _);
}

void Calculate(int x, int y, out int sum, out int diff)
{
    sum = x + y;
    diff = x - y;
}

Notice the “_” as the last parameter to Calculate method. Here, we do not intend to use the value of out diff and we replace a variable declaration with an “_”.