A struct is normally mutable, i.e. you can modify the values of its members directly.
However, if a struct is used in a collection class, like a List<T>, you can’t modify its members. Referencing the item by indexing into the collection returns a copy of the struct, which you can’t modify. To change an item in the list, you need to create a new instance of the struct.
List<DogCollarInfo> collarList = new List<DogCollarInfo>();
// Create a few instances of struct and add to list
collarList.Add(new DogCollarInfo(0.5, 14.0));
collarList.Add(new DogCollarInfo(0.3, 12.0));
// Compile-time error: Can't modify '...' because it's not a variable
collarList[1].Length = 22.0;
// Do this instead
collarList[1] = new DogCollarInfo(collarList[1].Width, 22.0);
If you store the structs in an array, then you can change the value of one of the struct’s members.
DogCollarInfo[] arr = new DogCollarInfo[2];
arr[0] = new DogCollarInfo(0.5, 14.0);
arr[0].Length = 5.0; // OK