What data type should the argument of the Equals method have?

The argument of the Equals method in C# must be of type object. This allows the method to accept any data type, as all types in C# inherit from the base class object. Usually, the method looks like this:

public override bool Equals(object obj)
{
    if (obj == null || GetType() != obj.GetType())
        return false;
    // compare fields
}