我可以从扩展方法返回不同的类型

Question

I basically want to write a DataTable extension method to hide a generalized LINQ query. Lets say I have the DataTables below:

DataTable1

[ID]    [NameOfThing]    [DateOfThing]
1       'FirstName'      1/2/34
2       'SecondName'     5/6/78

DataTable2

[SomeFieldId]    [NumberOfItems]
3                934
4                20393

I want to be able to call an extension method like string aString = DataTable1.GetDataItem(1,"ID","NameOfThing")
or
int anInt = DataTable2.GetDataItem(3,"SomeFieldId","NumberOfItems")

I think my extension method would look something like this:

public static var GetDataItem(this DataTable dTable, int idToMatch
                          ,string fieldToMatchIdTo, string fieldToReturn)  
{  
    DataRow [] results = dTable.AsEnumerable()  
          .Where(r => r.Field`<int>`(fieldToMatchTo) == idToMatch).ToArray();  
    return (var)results[0][fieldToReturn];  
}

I know there's a lot missing here (casting to var seems dubious) but hopefully I captured the idea of what I'm trying to do. Is it possible, or are there any other good alternatives? I could write an extension method for each return type I suppose but I'm hoping there's a more generalized approach out there.

Answer 1

You can just make it generic:

public static T GetDataItem<T>(this DataTable dTable, int idToMatch, string fieldToMatchIdTo, string fieldToReturn)  
{  
    DataRow results = dTable.AsEnumerable()  
          .Where(r => r.Field<int>(fieldToMatchTo) == idToMatch).ToArray();  
    return (T)results[0][fieldToReturn];  
}

which you can call like:

int anInt = DataTable2.GetDataItem<int>(3,"SomeFieldId","NumberOfItems");

if for some reason you don't know the type of the returned value, you'll have to just return object.

Note that since you expect at least one row to match you can use First instead of Where :

DataRow row = dTable.AsEnumerable().First(r => r.Field<int>(fieldToMatchTo) == idToMatch);
return (T)row[fieldToReturn];

Answer 2

You can change your method like this to use generics

 public static T GetDataItem<T>(this DataTable dTable, int idToMatch
                      ,string fieldToMatchIdTo, string fieldToReturn)  
 {  
DataRow results = dTable.AsEnumerable()  
      .Where(r => r.Field`<int>`(fieldToMatchTo) == idToMatch).ToArray();  
return (T)results[0][fieldToReturn];  
}

and your calls will be like this

int result = DataTable2.GetDataItem<int>(3,"SomeFieldId","NumberOfItems")
string aString = DataTable1.GetDataItem<string>(1,"ID","NameOfThing")

Answer 3

I would probably make a generic method like this:

public static T GetDataItem<T>(this DataTable dTable, int idToMatch, 
                               string fieldToMatchIdTo, string fieldToReturn)  
{  
    var result = dTable.AsEnumerable()  
                       .Where(r => r.Field<int>(fieldToMatchTo) == idToMatch)
                       .FirstOrDefault();

    if (result == null)
    {
        return default(T);
    }
    else
    {
        return result.Field<T>(fieldToReturn);
    }
}

Usage:

var anInt = DataTable2.GetDataItem<int>(3,"SomeFieldId","NumberOfItems")