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In C# readonly allows initializing and modifying the value in constructor.

In addition, a const of a reference type other than string can only be initialized with null (hence, const only make sense if of a value type or string). And only data members (class level variables) can be readonly.

still i'm not clear...someone explain the difference clearly...thank you.

C# Readonly allows you to initialize variable in Constructor, whereas Constant value should be specified at compile time. Constant values may be stored in register as optimization whereas Readonly variables are allocated memory in same way as any normal variable

linke - stackoverflow.com/questions/55984/what-is-the-difference-between-const-and-readonly

Constant is a compile-time constant, and read-only is runtime constant.

1) A const cannot be static,while readonly can be static.
2) A const need to be declared and initialized at declaration only,while a readonly can be initialized at declaration or by the code in the constructor.
3) A const's value is evaluated at design time,while a readonly's value is evaluated at runtime.

Constant is a compile-time constant and will be inlined by the compile. This is why constants should be private or internal to avoid dependecy issues. It can contain only value types, except of string and null.

A readonly will be initialized either at its declaration or within the constructor and is not restricated to value types. Its value can only be changed via reflection.