The fixed keyword in C# instructs the garbage collector from reallocating memory to the specified variable. However, fixed can only be used in an unsafe context.
To access a member of an object that is fixed, we need to declare an appropriate fixed pointer variable to first store the reference to that member (also known as pinning fixed fields), and then we can manipulate it. This is done to ensure that the garbage collector does not reallocate our pointer variable. However, in C# 7.3 onward, we don’t need to declare a fixed pointer to access a member of a fixed object.
The example below shows how the fixed keyword was used to index fields using pinning prior to C# 7.3:
unsafe struct Temp {public fixed int arr[5];}class Program {static void Main() {Temp t = new Temp();unsafe {for (int i = 0; i < 5; i++) {fixed (int * arrP = t.arr) {arrP[i] = i + 1;}}for (int i = 0; i < 5; i++) {fixed (int * arrP = t.arr) {System.Console.WriteLine(arrP[i]);}}}}}
Output:
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3
4
5
In the example above, an unsafe struct Temp is declared that contains a fixed integer array arr. In the Main function, we create an instance of Temp named t. Now, in an unsafe context, we have two for loops to iterate over the array t.arr. The first for loop initializes the array with the first 5 digits and the second for loop prints this array onto the console. However, to access an element of t.arr (which is a fixed array), we need to declare a fixed pointer of type int; this is called pinning fixed fields. Afterward, using this pointer arrP, we are able to index the array.
The following example rewrites the previous example based on C# 7.3 improvements:
unsafe struct Temp {public fixed int arr[5];}class Program {static void Main() {Temp t = new Temp();unsafe {for (int i = 0; i < 5; i++) {t.arr[i] = i + 1;}for (int i = 0; i < 5; i++) {System.Console.WriteLine(t.arr[i]);}}}}
Output:
1
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5
In the example above, we no longer need to pin the fixed array arr and index it. We simply index arr as we usually do with other non-fixed arrays.
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