"Ref" and "Out "keywords in C#

Question

Hi................ I am not understand what is a used of "ref" and "out" keywords of c# ? please expalin and give a example.

Datta Kharad · Answer

Hi Ref and Out:- Out is used whenever you want to expect only output value and doesn't send input. Ref is used whenever you want to expect output value according to input i,e send input and get output using single variable. Ref parameters allow you to pass variable reference parameters, allowing you to assign the parameter in the called method and have it also be assigned at the calling site. This is powerful and useful but also has disadvantages. Here we look at how you can use the ref and out keywords on parameters. A variable passed with ref must be assigned first. One with out must be assigned in the called method. Example There is a big difference between ref and out parameters at the language level. Before you can pass a ref parameter, you must assign it to a value. This is for the purpose of definite assignment analysis. On the other hand, you don't need to assign an out parameter before passing it to a method. However, the out parameter in that method must assign the parameter before returning. using System; class Program { static void Main() { string value1 = "book"; // Assign string value SetString1(ref value1); // Pass as reference parameter Console.WriteLine(value1); // Write result string value2; // Unassigned string SetString2(1, out value2); // Pass as out parameter Console.WriteLine(value2); // Write result } static void SetString1(ref string value) { if (value == "book") // Test parameter value { Console.WriteLine("Is book"); } value = "notebook"; // Assign parameter to new value } static void SetString2(int number, out string value) { if (number == 1) // Check int parameter { value = "one"; // Assign out parameter } else { value = "pen"; // Assign out parameter } } } //Output Is book notebook one Description. The program in the example defines the Program class with the Main entry point and two static methods, SetString1 and SetString2. The methods have formal parameter lists with the reference parameter keywords. SetString1 method with ref string. The SetString1 method shown above has the "ref" keyword in its method signature. This requires that whenever you pass a string to this method, it must be described with the "ref" keyword before the identifier. When the method is called, the storage location of the string variable is copied to the method. None of the characters pointed to by the string variable are copied. SetString2 method with out string. The SetString2 method has the "out" keyword in its signature. This means that whenever you want to call it, you must describe the string parameter with the "out" keyword. You do not need to assign the string parameter before sending it to SetString2. However, you will get a compile-time error if you do not assign the string parameter before any return points.

Maha · Answer

Neither reference nor output parameters occupy their own memory locations. Rather, both reference and output parameters act as aliases, or other names, for the same memory location occupied by the original passed variable. When you use a reference parameter, the passed variable must have an assigned value. Reference Parameter using System; public class ParameterDemo2 { public static void Main() { int var = 4; Console.WriteLine("In Main var is {0}", var); MethodWithRefParam(ref var); Console.WriteLine("In Main var is {0}", var); } public static void MethodWithRefParam(ref int parm) { parm = 888; Console.WriteLine("In MethodWithRefParam, param is {0}", parm); } } /* In Main var is 4 In MethodWithRefParam, param is 888 In Main var is 888 (Notice value has been alered 4 to 888) */ Using an output parameter is convenient when the passed variable doesn't have a value yet. For example, the below program uses InputMethod() to obtain values for two parameters. The parameters get their values from the method, so it makes sense to provide them with no values going in, but to have them retain values coming out using System; public class InputMethodDemo { public static void Main() { int first, second; InputMethod(out first, out second); Console.WriteLine("After InputMethod first is {0} and second is {1}", first, second); } public static void InputMethod(out int one, out int two) { string s1, s2; Console.Write("Enter first integer "); s1 = Console.ReadLine(); one = Convert.ToInt32(s1); Console.Write("Enter second integer "); s2 = Console.ReadLine(); two = Convert.ToInt32(s2); } } /* Enter first integer 2 Enter second integer 4 After InputMethod first is 2 and second is 4 */

Vulpes · Answer

As we already have a large number of articles on these keywords (see Recommended Articles to right of this thread), I'll try and explain the difference using an example: using System; class Test { static void Main() { // value types int i = 0; MyMethod(ref i); Console.WriteLine(i); // now 2 int j ; // not necessary for variable passed as 'out parameter to be assigned to first MyMethod2(out j); Console.WriteLine(j); // now 3 // reference types MyClass obj = new MyClass(); MyMethod3(ref obj); Console.WriteLine(obj.K); // now 5 MyClass obj2; // not necessary for variable passed as 'out parameter to be assigned to first MyMethod4(out obj2); Console.WriteLine(obj2.K); // still 4, the intial value on construction Console.ReadKey(); } static void MyMethod(ref int i) { i = 2; // any changes made to 'ref' parameter will be persisted when method return } static void MyMethod2(out int i) { i = 3; // must assign to 'out' parameter before the method returns } static void MyMethod3(ref MyClass mc) { mc.K = 5; // can change state of 'ref' parameter before the method returns } static void MyMethod4(out MyClass mc) { mc = new MyClass(); // must assign to 'out' parameter before the method returns } } class MyClass { public int K = 4; }

Hemant Kumar · Answer

Ref tells the compiler that the object is initialized before entering the function, while out tells the compiler that the object will be initialized inside the function. So while ref is two-ways, out is out-only.

"Ref" and "Out "keywords in C#

Answers (4)