C# Passing Generic Class as a parameter to a custom Method

Question

I'm trying to create a generic function that can take in the following parameters.

1. Class Type - classType
2. Page Number - pageNum
3. Page Size - pageSize
4. Order by Column - orderByColumn

Below is an example of the function that I'm trying to replicate.

 public List<T> GetDataPerPage<T>(IList<T> classType, int pageNum, int pageSize, string orderByColumn)
       {
        if (pageSize <= 0) pageSize = 10;  // TODO: Default pageSize for the Moment
        if (pageNum <= 0) pageNum = 1;
        int excludedRows = (pageNum - 1) * pageSize;

        return GetRepo<classType>().All(null).AsQueryable().Where(p => p.IsDeleted == false).OrderBy(p => p.orderByColumn).Skip(excludedRows).Take(pageSize).ToList();                                  
    }

I'm uncertain of how it should pass the classType and the orderByColumn to the function.

Any help would be greatly appreciated. Thanks

Answer 1

You do not need to pass the classType parameter at all, because you will declare the type T when you use the GetDataPerPage<T>() method, so change it to this:

public List<T> GetDataPerPage<T>(int pageNum, int pageSize, string orderByColumn)
{
    if (pageSize <= 0) pageSize = 10;  // TODO: Default pageSize for the Moment
    if (pageNum <= 0) pageNum = 1;
    int excludedRows = (pageNum - 1) * pageSize;

    return GetRepo<T>().All(null)
                       .AsQueryable()
                       .Where(p => p.IsDeleted == false)
                       .OrderBy(p => p.orderByColumn)
                       .Skip(excludedRows)
                       .Take(pageSize)
                       .ToList();                                  
}

---

As for the orderByColumn parameter, if you are going to just do a comparison like your example, then passing in a string is fine, but if you want to control the logic of the comparison to use a string property of type T , then pass a delegate like this:

public List<T> GetDataPerPage<T>(int pageNum, int pageSize, Func<T, string> orderBy)
{
    if (pageSize <= 0) pageSize = 10;  // TODO: Default pageSize for the Moment
    if (pageNum <= 0) pageNum = 1;
    int excludedRows = (pageNum - 1) * pageSize;

    return GetRepo<T>().All(null)
                       .AsQueryable()
                       .Where(p => p.IsDeleted == false)
                       .OrderBy(orderBy)
                       .Skip(excludedRows)
                       .Take(pageSize)
                       .ToList();                                  
}

Usage:

var list = GetDataPerPage<T>(pageNum, pageSize, p => p.Property1);

Answer 2

You already know the "class type".. its T :

GetRepo<T>().All(null)...

For the OrderBy .. pass in a delegate yourself:

public List<T> GetDataPerPage<T>(IList<T> classType, 
                                 int pageNum, 
                                 int pageSize, 
                                 Func<T, object> orderBy)
{
    ...

    return ... ... .OrderBy(orderBy)
}

And pass the delegate in like this:

GetDataPerPage(..., ..., ..., x => x.SomePropertyHere);