Why does 'only assignment, call, increment, decrement, await, and new object expressions can be used as a statement' error occur in c#?

This error appears when you use an expression that doesn't do anything meaningful on its own.

Example:

int x;
x + 1;

Solution:

int x;
x = x + 1;

Always ensure that expressions in your code are doing something useful, such as being assigned to a variable.