C# Language out var declaration


Example

A common pattern in C# is using bool TryParse(object input, out object value) to safely parse objects.

The out var declaration is a simple feature to improve readability. It allows a variable to be declared at the same time that is it passed as an out parameter.

A variable declared this way is scoped to the remainder of the body at the point in which it is declared.

Example

Using TryParse prior to C# 7.0, you must declare a variable to receive the value before calling the function:

7.0
int value;
if (int.TryParse(input, out value)) 
{
    Foo(value); // ok
}
else
{
    Foo(value); // value is zero
}

Foo(value); // ok

In C# 7.0, you can inline the declaration of the variable passed to the out parameter, eliminating the need for a separate variable declaration:

7.0
if (int.TryParse(input, out var value)) 
{
    Foo(value); // ok
}
else
{
    Foo(value); // value is zero
}

Foo(value); // still ok, the value in scope within the remainder of the body

If some of the parameters that a function returns in out is not needed you can use the discard operator _.

p.GetCoordinates(out var x, out _); // I only care about x

An out var declaration can be used with any existing function which already has out parameters. The function declaration syntax remains the same, and no additional requirements are needed to make the function compatible with an out var declaration. This feature is simply syntactic sugar.

Another feature of out var declaration is that it can be used with anonymous types.

7.0
var a = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
var groupedByMod2 = a.Select(x => new
                                  {
                                      Source = x,
                                      Mod2 = x % 2
                                  })
                     .GroupBy(x => x.Mod2)
                     .ToDictionary(g => g.Key, g => g.ToArray());
if (groupedByMod2.TryGetValue(1, out var oddElements))
{
    Console.WriteLine(oddElements.Length);
}

In this code we create a Dictionary with int key and array of anonymous type value. In the previous version of C# it was impossible to use TryGetValue method here since it required you to declare the out variable (which is of anonymous type!). However, with out var we do not need to explicitly specify the type of the out variable.

Limitations

Note that out var declarations are of limited use in LINQ queries as expressions are interpreted as expression lambda bodies, so the scope of the introduced variables is limited to these lambdas. For example, the following code will not work:

var nums = 
    from item in seq
    let success = int.TryParse(item, out var tmp)
    select success ? tmp : 0; // Error: The name 'tmp' does not exist in the current context

References