EntityFunction.cs

[FunctionName("Counter")]
public static async Task Counter(
    [EntityTrigger] IDurableEntityContext ctx)
{
    int currentValue = ctx.GetState<int>();
    int operand = ctx.GetInput<int>();

    switch (ctx.OperationName)
    {
        case "add":
            currentValue += operand;
            break;
        case "subtract":
            currentValue -= operand;
            break;
        case "reset":
            await SendResetNotificationAsync();
            currentValue = 0;
            break;
    }

    ctx.SetState(currentValue);
}