Command Query Separation (CQS) in C#

CQS is the principle of development where we need to separate command from the query. It's a basic rule that a developer must follow. Segregation of command with the query is very important.

Command:
Which change the state of a system. In event-driven programming, the function that affects the system's state is a command.

The method whose return type is void is supposed to be command type.

Example:
A button click event in web form is a command as it changes the state of the system.

protected void register_Click(object sender, EventArgs e) {
    // your code to registeration
}

Query : 

The query can be defined as a method that has a return type. A query must return something. This is like you are asking something from the system.

Example: 

public bool IsValidUserName(string UserName) {
    bool validUser = false;
    //code to check user from db
    //if user exist set validUser=true else false
    return validUser;
}

This method is used to check the valid user for UserName parameter and return status accordingly. 

You can add the logic, the motive is to check the user is valid or not if valid than return true else false.