Getting a column by string name

Question

I'm trying to update a record given the customer Id, the row Id, and a dynamic column name.

Thus far I have the following, with the trouble spot marked by ***:

    public void UpdateRecord(int Id, string rval, string column, string value)
    {
        var rId = GetRvalId(rval);
        var entry = _context.Customers
            .Where(x => x.Id == Id && x.RVals.Id == rId && x.***column?*** == column).First();
        entry = value;
    }

I haven't been able to find a good example of how to do this.

Answer 1

Addition after comments at the end

The reason you couldn't find examples is because it is not a good design.

Your method is very error prone, difficult to test and horrible to maintain. What if someone types the incorrect column name? What if you try to assign a string to the customer's birthday? And even if you would implement some string checking for column names and proposed values, then your program wouldn't work anymore after someone changes the names or the types of the columns.

So let's redesign!

Apparently you have a Customer with an Id and a property Rvals. This property Rvals also has a property Id.

You also have a function GetRValId that can convert a string rval to an int rvalId.

What you want, is given an Id and a string rval, you want to update one of the columns of the first Customer with this Idand rValId.

Side questions: Can there be more than one Customer with Id? In that case: are you sure Id is an ID? What do you want if there are more matching Customers? Update all customers or update only the first one? Which customer do you define as the first customer?

Leaving the side questions aside. We want a function signature that reports errors at compile time if you use non-existing customer properties, or if you try to assign a string to a Birthday. Something like this perhaps?

Update the name of the customer:

int customerId = ...
string rval = ...
string proposedName = "John Doe";
UpdateCustomerRecord(id, rval, customer => customer.Name = proposedName);

Update the Birthday of the customer:

DateTime proposedBirthday = ...
UpdateCustomerRecord(id, rval, customer => customer.Birthday = proposedBirthday)

This way you can't use any column that does not exist, and you can't assign a string to a DateTime.

You want to change two values in one call? Go ahead:

UpdateCustomerRecord(id, rval, customer =>
{
    customer.Name = ...;
    customer.Birthday = ...;
});

Convinced? Let's write the function:

public void UpdateCustomerRecord(int customerId, string rval, Action<Customer> action)
{
    // the beginning is as in your function:
    var rId = GetRvalId(rval);

    // get the customer that you want to update:
    using (var _Context = ...)
    {
        // get the customer you want to update:
        var customerToUpdate = _Context.Customers
            .Where(customer => customer.Id == Id
                && customer.RVals.Id == rId)
            .FirstOrDefault();

        // TODO: exception if there is no customerToUpdate

        // perform the action and save the changes
        action(customerToUpdate);
        _context.SaveChanges();
}

Simple comme bonjour!

Addition after comments

So what does this function do? As long as you don't call it, it does nothing. But when you call it, it fetches a customer, performs the Action on the Customer you provided in the call, and finally calls SaveChanges.

It doesn't do this with every Customer, no it does this only with the Customer with Id equal to the provided Id and customer.RVals.Id == ... (are you still certain there is more than one customer with this Id? If there is only one, why check for RVals.Id?)

So the caller not only has to provide the Id, and the RVal, which define the Customer to update, but he also has to define what must be done with this customer.

This definition takes the form of:

customer =>
{
    customer.Name = X;
    customer.BirthDay = Y;
}

Well if you want, you can use other identifiers than customer, but it means the same:

x => {x.Name = X; x.BirthDay = Y;}

Because you put it on the place of the Action parameter in the call to UpdateCustomerRecord, I know that x is of type Customer.

The Acton statement means: given a customer that must be updated, what must we do with the customer? You can read it as if it was a Function:

void Action(Customer customer)
{
    customer.Name = ...
    customer.BirthDay = ...
}

In the end it will do something like:

Customer customerToUpdate = ...
customerToUpdate.Name = X;
customerToUpdate.BirthDay = Y;
SaveChanges();

So in the third parameter, called Action you can type anything you want, even call functions that have nothing to do with Customers (probably not wise). You have an input parameter of which you are certain that it is a Customer.

See my earlier examples of calling UpdateCustomerRecord, one final example:

UpdateCustomerRecord( GetCustomerId(), GetCustomerRVal,
   // 3rd parameter: the actions to perform once we got the customerToUpdate:
   customer =>
   {
       DateTime minDate = GetEarliestBirthDay();
       if (customer.BirthDay < minDate)
       {   // this Customer is old
           customer.DoThingsThatOldPeopleDo();
       }
       else
       {   // this Customer is young
           customer.DoThingsThatYoungPeopleDo();
       }
   }
}

So the Action parameter is just a simpler way to say: "once you've got the Customer that must be updated, please perform this function with the Customer

So if you only want to update a given property of the customer write something like:

UpdateCustomerRecord(... , customer =>
{
     Customer.PropertyThatMustBeUpdated = NewValueOfProperty;
}

Of course this only works if you know which property must be updated. But since you wrote "I am trying to update a specific cell." I assume you know which property the cells in this column represent.

Answer 2

It is not possible to pass the column name as the string value in LINQ. Alternate way to do it, if you have the limited number of the column name which can be passed then it can be achieved as below:

public void UpdateRecord(int Id, string rval, string column, string value)
{
    var rId = GetRvalId(rval);
    var entry = _context.Customers
        .Where(x => x.Id == Id && 
               x.RVals.Id == rId && 
              (x.column1 == value || column == column1) &&
              (x.column2 == value || column == column2) && 
              (x.column3 == value || column == column3) &&
              (x.column4 == value || column == column4) &&
              (x.column5 == value || column == column5) &&
               )).First();
    entry = value;
}

UpdateRecord(5, "rval", "column1", "value");
UpdateRecord(5, "rval", "column2", "value");
UpdateRecord(5, "rval", "column3", "value");

Here, suppose you have the 5 columns that can be passed while calling the funcion UpdateRecord then you can add the 5 clauses in the WHERE as above.

Other way to do it dynamic LINQ

var entry = db.Customers.Where(column + " = " + value).Select(...);