Parameter Passing in C#

Parameter Arrays

Output parameters

Like reference parameters, output parameters don't create a new storage location, but use the storage location of the variable specified on the invocation. Output parameters need the out modifier as part of both the declaration and the invocation - that means it's always clear when you're passing something as an output parameter.

Output parameters are very similar to reference parameters. The only differences are:

  • The variable specified on the invocation doesn't need to have been assigned a value before it is passed to the function member. If the function member completes normally, the variable is considered to be assigned afterwards (so you can then "read" it).
  • The parameter is considered initially unassigned (in other words, you must assign it a value before you can "read" it in the function member).
  • The parameter must be assigned a value before the function member completes normally.

Here is some example code showing this, with an int parameter (int is a value type, but if you understood reference parameters properly, you should be able to see what the behaviour for reference types is):

void Foo (out int x)
    // Can't read x here - it's considered unassigned
    // Assignment - this must happen before the method can complete normally
    x = 10;
    // The value of x can now be read:
    int a = x;
// Declare a variable but don't assign a value to it
int y;

// Pass it in as an output parameter, even though its value is unassigned
Foo (out y);
// It's now assigned a value, so we can write it out:
Console.WriteLine (y);


Parameter arrays

Parameter arrays allow a variable number of arguments to be passed into a function member. The definition of the parameter has to include the params modifier, but the use of the parameter has no such keyword. A parameter array has to come at the end of the list of parameters, and must be a single-dimensional array. When using the function member, any number of parameters (including none) may appear in the invocation, so long as the parameters are each compatible with the type of the parameter array. Alternatively, a single array may be passed, in which case the parameter acts just as a normal value parameter. For example:

void ShowNumbers (params int[] numbers)
    foreach (int x in numbers)
        Console.Write (x+" ");

int[] x = {1, 2, 3};
ShowNumbers (x);
ShowNumbers (4, 5);


In the first invocation, the variable x is passed by value, as it's just an array. In the second invocation, a new array of ints is created containing the two values specified, and a reference to this array is passed.


Some informal definitions and summaries of terms:

Function member
A function member is a method, property, event, indexer, user-defined operator, instance constructor, static constructor, or destructor.
Output parameter
A parameter very similar to a reference parameter, but with different definite assignment rules.
Reference parameter (pass-by-reference semantics)
A parameter which shares the storage location of the variable used in the function member invocation. As they share the same storage location, they always have the same value (so changing the parameter value changes the invocation variable value).
Reference type
Type where the value of a variable/expression of that type is a reference to an object rather than the object itself.
Storage location
A portion of memory holding the value of a variable.
Value parameter (the default semantics, which are pass-by-value)
A value parameter that has its own storage location, and thus its own value. The initial value is the value of the expression used in the function member invocation.
Value type
Type where the value of a variable/expression of that type is the object data itself.
Name associated with a storage location and type. (Usually a single variable is associated with a storage location. The exceptions are for reference and output parameters.)

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Jon Skeet United Kingdom

C# MVP currently living in Reading and working for Google.

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