The following table lists the instruction's hexadecimal and Microsoft intermediate language (MSIL) assembly format, along with a brief reference summary:

This prefix can only appear immediately preceding the ldelema instruction and calls to the special Address method on arrays. Its effect on the subsequent operation is twofold:

The pointer is said to have restricted mutability because the defining type controls whether the value can be mutated. For value classes that expose no public fields or methods that update the value in place, the pointer is read-only (hence the name of the prefix). In particular, the classes representing primitive types (for example, System.Int32) do not expose mutators and thus are read-only.

A managed pointer restricted in this fashion can be used only in the following ways:

• As the object parameter for the ldfld, ldflda, stfld, call, or constrained callvirt instructions.

• As the pointer parameter to the ldobj instruction or to one of the ldind instructions.

• As the source parameter to the cpobj instruction.

All other operations disallowed, including the stobj, initobj, or mkrefany operations, or any of the stind instructions.

The purpose of the readonly prefix is to avoid a type check when fetching an element from an array in generic code. For example, the expression arr[i].m(), where the element type of the array arr is a generic type that has been constrained to have an interface with method m, might compile to the following MSIL.

ldloc arr
ldloc i
readonly.
ldelema !0    // Loads the pointer to the object.
…             // Load the arguments to the call.
constrained. !0
callvirt m

Without the readonly prefix, the ldelema instruction would perform a type check in the case where !0 was a reference type. Not only is this type check inefficient, but it is semantically incorrect. The type check for ldelema is an exact match, which is too strong. If the array held subclasses of type !0, the code above would fail the type check.

The address of the array element is fetched, instead of the element itself, in order to have a handle for arr[i] that works for both value types and reference types, and thus can be passed to the constrained callvirt instruction.

In general it would be unsafe to skip the run-time check if the array held elements of a reference type. To be safe, it is necessary to ensure that no modifications to the array are made through this pointer. The verifier rules ensure this. The restricted managed pointer can be passed as the object of instance method calls, so it is not strictly speaking read-only for value types, but there is no type safety problem for value types.

The following ILGenerator.Emit(OpCode) method overload can use the readonly opcode:

• ILGenerator.Emit(OpCode)