use-ref-arguments.md

Use ref instead of values

Let's start by modifying the Find method to return a reference to an element in matrix. Change the return type of the Find method from (int i, int j) to ref int.

After that change, you'll need to modify both return statements. The first should return the element instead of the indices:

return matrix[i,j];

Note this doesn't compile. You need to add the ef modifier to the return statement. This makes it clear that the code returns a reference, not a value.

return ref matrix[i,j];

The second return should throw an exception instead of returning invalid indices, because you can't return a reference to a "not found" element:

throw new InvalidOperationException("Not found");

The Find method now returns a reference to the found element. This introduces compiler errors in the code that tests this method. Replace the last three lines of the code below with the following code:

var valItem = Find(sourceMatrix, (val) => val == 42);
Console.WriteLine(valItem);
valItem = 24;
Console.WriteLine(sourceMatrix[4, 2]);

Try this code. The value in sourceMatrix doesn't change. That's because valItem is not declared as a ref local variable. Because it's declared as a value, the return value is copied. Add the ref modifier before var in the declaration of valItem. That introces a new compiler error. Add the ref modifier before the Find method name to fix it:

ref var valItem = ref Find(sourceMatrix, (val) => val == 42);

When first introduced, you couldn't reassign the reference that a ref local variable refers to. Beginning with 7.3, you can reassign a ref local variable. Add the following code to the bottom of the test code:

valItem = Find(sourceMatrix, (val) => val == 16);

The C# language has three other rules that protect you from misusing the ref locals and returns:

• You cannot assign a standard method return value to a ref local variable.
• You cannot return a ref to a variable whose lifetime does not extend beyond the execution of the method.
• ref locals and returns can't be used with async methods.

The addition of ref locals and ref returns enable algorithms that are more efficient by avoiding copying values or performing dereferencing operations multiple times.

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