ref Keyword
This page was last reviewed on May 29, 2023.
Dot Net Perls
Ref. A C# ref parameter is passed as a reference, not a value. This means we can assign the parameter in the called method and have it also be assigned at the calling site.
There is a difference between ref and out. Unlike with "out," we do not need to assign an initial value to ref before passing it to a method.
This program defines the Program class with Main and 2 other methods—SetString1 and SetString2. The methods have formal parameter lists with ref keywords.
Part 1 This code uses the ref keyword. Whenever we pass a string to this method, it must have the ref keyword.
Part 2 This code has the out keyword. Whenever we want to call SetString2, we must use the out keyword on its argument.
Tip We do not need to assign the string parameter before sending it to SetString2.
using System; class Program { static void Main() { // Part 1: use ref argument. string value1 = "cat"; SetString1(ref value1); Console.WriteLine(value1); // Part 2: use out argument. string value2; SetString2(1, out value2); Console.WriteLine(value2); } static void SetString1(ref string value) { if (value == "cat") { Console.WriteLine("Is cat"); } value = "dog"; } static void SetString2(int number, out string value) { if (number == 1) { value = "one"; } else { value = "carrot"; } } }
Is cat dog one
Example 2. We can refactor code that acts upon multiple local variables or fields. We create a method with a ref argument. And then we pass each separate variable to this method.
Warning Using a loop for this pattern may be clearer and faster. If your program happens to use separate fields, though, this can help.
Note The ref argument in the NullIf method both reads and writes the string. It is an input and output argument.
using System; class Program { static void Main() { string v1 = "dot"; string v2 = "net"; string v3 = "perls"; NullIf(ref v1); NullIf(ref v2); NullIf(ref v3); Console.WriteLine(v1); Console.WriteLine(v2); Console.WriteLine(v3); } static void NullIf(ref string value) { if (value == "net") { value = null; } } }
dot perls
Ref returns. A method can return a reference to an argument or a local. This is useful if we want to modify the returned object, and have these modifications reflected in the current scope.
Here We pass 2 Test structs to the HigherValue method. It compares the structs, and returns a reference to one of them.
Detail We have 2 local variables and we pass references of these to HigherValue. We modify the returned struct.
Finally The modification affects the local variables in the Main method. So the ref return affects the local state of Main.
using System; struct Test { public int Value; } class Program { static ref Test HigherValue(ref Test left, ref Test right) { // Compares the two Test struct arguments. // ... Return a reference to the one with the higher value. if (left.Value > right.Value) { return ref left; } else { return ref right; } } static void Main() { Test t1; t1.Value = 10; Test t2; t2.Value = 20; // Get the struct with the higher value. // ... Then modify its value. HigherValue(ref t1, ref t2).Value = 30; // Display values of 2 structs. Console.WriteLine(t1.Value); Console.WriteLine(t2.Value); } }
10 30
Ref returns, notes. To use a ref return to our advantage, a mutable struct (or array) is needed. We can assign to the result of a method—this effect is reflected in the current method.
Implementation. The difference between ref and out is in the C# language itself. In the intermediate language, we find that the ref and out parameters are called with the "string" type.
Note The "string" type aliases the System.String type in the base class library, so it is not special-cased here.
A warning. Ref and out can confuse the overload resolution step in the C# compiler. For this reason, it is a bad idea to mix ref and out parameters in an interface or object method group.
Uses. Out is sometimes used in methods that enhance performance—it can lead to simpler code. But often it makes code more complex and should be avoided.
Dictionary TryGetValue
The ref keyword allows us to pass variable references, as opposed to object references. The storage location of the variable itself is copied on the method invocations.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on May 29, 2023 (edit).
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