Positional and keyword parameters

In C#, arguments are bound to parameters based on their position (or order) by default; but C# also supports keyword parameters through the use of named arguments (Docs, 2015).

Take this Add method as an example:

int Add(int first, int second, int third)
{
    return first + second + third;
}

Positional

This is the default way of how arguments and parameters are bound to each other:

void Main()
{
    int result = 0;
    result = Add(1, 2, 3);
    Console.WriteLine(result);
}

Named

The caller can specify the value of a parameter by associating the argument with the parameter’s name in the following format:

<parameter name>: <argument value>

The same method previously can also be invoked with:

result = Add(first: 1, second: 2, third: 3);

or even out-of-order:

result = Add(third: 3, second: 2, first: 1);

However, named arguments must be placed before any fix arguments. Thus the following code is invalid:

// ERROR: CS8323 Named argument is used out-of-position but is followed by an unnamed argument
result = Add(third: 3, 2, 1);

Live-code example

Named arguments are commonly used to convey the intent of the argument. Without looking at the definition of the method, which one is better?

Init(true, false, true);

vs.

Init(isProcessDataRequired: true,
     isDiplayDataRequired: false,
     isSaveDataRequired: true);

Optional

Optional arguments allow the caller to omit arguments for some parameters. The omitted parameter will use the default value defined in the method header.

For example:

void Print(string required, string optionalString = "Default string", int optionalNumber = 123)
{
    Console.WriteLine($"{required}, {optionalString}, {optionalNumber}");
}

Can be called with:

Print("One");
Print("One", "Two");
Print("One", "Two", 3);

Output:

One, Default string, 123
One, Two, 123
One, Two, 3

By default, the arguments are positional, but named arguments can also be used to specify the value of the optional parameter.

For example, to skip the second parameter optionalString:

Print("One", optionalNumber: 3);

Output:

One, Default string, 3

Live-code example

References

Docs, M. (2015, July 20). Named and Optional Arguments (C# Programming Guide). Retrieved from https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/named-and-optional-arguments

C# - TPL2141

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