How to best check that variables have values in C#?

See more:

C#

WinForms

controller

Nullable

I am a novice programmer. I have a few single value return "gets" that return ID's that looks like this:

Controller.GetCustomerIDByCustomerName(item.Customer.ToLower().Trim(), out int CustomerID);

                    Controller.GetCustomerIDByCustomerName(item.FinalCustomer.ToLower().Trim(), out int FinalCustomerID);

                    Controller.GetHaulierIDByName(item.HaulierName.ToLower().Trim(), out int HaulierID);

                    Controller.GetCustomerDepotIDByName(item.DepotName.ToLower().Trim(), CustomerID, out int CustomerDepotID);

I want to check that CustomerID/ FinalCustomerID etc... have value before continuing with the code. I'm not sure what the best/ most efficient way to do this is.

What I have tried:

I have tried a method that checks on the data:

private bool ValidData()
{
    bool ReturnValue = true;

    if (CustomerID == null)
    {
        cGlobal.ShowWarningMessage("Customer Required", "Customer Required");
        ReturnValue = false;
    }

    if (ReturnValue)
    {
        if (FinalCustomerID == null)
        {
            cGlobal.ShowWarningMessage("Final Customer Required");
            ReturnValue = false;
        }
    }

    if (ReturnValue)
    {
        if (HaulierID == null)
        {
            cGlobal.ShowWarningMessage("Haulier Required");
            ReturnValue = false;
        }
    }

    if (ReturnValue)
    {
        if (CustomerDepotID == null)
        {
            cGlobal.ShowWarningMessage("Customer Depot Required");
            ReturnValue = false;
        }
    }

    return ReturnValue;
}

and then declaring the variables at the top of the page like so:

public int? CustomerID;
        public int? FinalCustomerID;
        public int? HaulierID;
        public int? CustomerDepotID;

However this doesn't work.

Any advice on how to do this would be appreciated.