C# - throw Keyword Examples

Throw. This C# keyword generates (or translates) exceptions. By using a throw statement inside a catch block, we can change the resulting exception.

Exception

Notes, throw. We can throw a new exception. The throw statement is versatile—throw is a statement, but also a special kind of expression.

Example. We first look at 3 methods A, B, and C that use the throw statement in different ways. Method A uses a throw statement with no argument.

Info The code in method A can be thought of as a rethrow—it throws the same exception already being handled.

Next Method B throws a named exception variable. This is not a rethrow—it throws the same exception but changes the stack trace.

Also We can collect information about the exception if needed. Method C creates a new exception.

Tip We can use a throw statement in this way to implement custom error conditions.

using System; class Program { static void Main() { // Call method A, method B, and method C. // ... Comment out the first 1-2 method invocations. try { A(); B(); C(null); } catch (Exception ex) { Console.WriteLine(ex); } } static void A() { // Rethrow syntax. try { int value = 1 / int.Parse("0"); } catch { throw; } } static void B() { // Filtering exception types. try { int value = 1 / int.Parse("0"); } catch (DivideByZeroException ex) { throw ex; } } static void C(string value) { // Generate new exception. if (value == null) { throw new ArgumentNullException("value"); } } }

System.DivideByZeroException: Attempted to divide by zero. System.DivideByZeroException: Attempted to divide by zero. System.ArgumentNullException: Value cannot be null. Parameter name: value

Rethrow. Next we examine rethrows in the C# language. A rethrow must use a throw statement with no argument. If you use throw ex, then the TargetSite and StackTrace are changed.

Detail In this program, X() uses a rethrow statement. And Y() uses a throw ex statement.

Result We see in the results that when a rethrow is used, the exception TargetSite is in StringToNumber—an internal method of int.Parse.

int.Parse

But When a throw ex is used, as in Y(), the exception's TargetSite was modified to the current method Y().

using System; class Program { static void Main() { try { X(); } catch (Exception ex) { Console.WriteLine(ex.TargetSite); } try { Y(); } catch (Exception ex) { Console.WriteLine(ex.TargetSite); } } static void X() { try { int.Parse("?"); } catch (Exception) { throw; // [Rethrow construct] } } static void Y() { try { int.Parse("?"); } catch (Exception ex) { throw ex; // [Throw captured ex variable] } } }

Void StringToNumber(System.String, ...) Void Y()

Throw expression. Throw is usually used as a statement. But in some places, it can be used as an expression. We can use throw as part of a null-coalescing statement.

Here Consider the Test() method. If its argument is null, a new ArgumentException is thrown as part of the null-coalescing statement.

using System; class Program { static void Test(string argument) { // Use null coalescing operator with throw expression. // ... If argument is null, throw. var value = argument ?? throw new ArgumentException("Bad argument"); Console.WriteLine("HERE"); } static void Main() { Test("bird"); Test(null); } }

HERE Unhandled Exception: System.ArgumentException: Bad argument at Program.Test(String argument) in c:\users\...Program.cs:line 9 at Program.Main() in c:\users\...Program.cs:line 16

Throw, ternary expression. A throw expression can only be used in certain places in programs—not everywhere. We can use a throw expression as part of a ternary statement.

Ternary

Here The string's length is tested. If it has fewer than 10 characters, an Exception is thrown as part of the ternary.

using System; class Program { static void Main() { string test = "bird"; // Throw can be used in ternary. char tenth = test.Length >= 10 ? test[9] : throw new Exception("Not ten characters"); // Not reached. Console.WriteLine(tenth); } }

Unhandled Exception: System.Exception: Not ten characters at Program.Main() in c:\users\...Program.cs:line 9

A review. The exception handling mechanism exploits an alternative control pathway. The throw statement provides the ability to rethrow an exception or generate a new one.

We can use throw as a statement, or an expression in some places in our programs. Though the syntax is confusing at first, it becomes easier to use with familiarity.