How to use Field<T> with Type?

Question

I have a method that determines the min and max of a column in a DataTable:

public void GetMinMaxRange( DataTable data, string valueColumnName )
{
   var min = data.AsEnumerable().Min(m => m.Field<double>(valueColumnName));
   var max = data.AsEnumerable().Max(m => m.Field<double>(valueColumnName));
}

I want to refactor this to be:

public void GetMinMaxRange( DataTable data, string valueColumnName )
{
   DataColumn column = data.Columns[valueColumnName];
   var min = data.AsEnumerable().Min(m => m.Field<column.DataType>(valueColumnName));
   var max = data.AsEnumerable().Max(m => m.Field<column.DataType>(valueColumnName));
}

I need to determine the datatype and use it instead of hard-coding m.Field<double>. How to do this?

UPDATE As to why I want to calculate the difference between the min and the max

    public static double/decimal/int GetMinMaxRange<T>(DataTable data, 
          string valueColumnName) where T : IComparable<T>
    {
        DataColumn column = data.Columns[valueColumnName];
        var min = data.AsEnumerable().Min(m => m.Field<T>(valueColumnName));
        var max = data.AsEnumerable().Max(m => m.Field<T>(valueColumnName)); ;
        return max - min;
    }

Tao

Exactly why do you need it? Try to finish that method, you are not using min/max yet. – Flaring

@Henk - updated my question. Does that make sense? – Tao

You can write that last sample as public static T GetMinMaxRange<T>(...) ... which indeed makes sense but needs to spec T at compiletime. – Flaring

@Henk - yes, but I can't seem to subtract min from max in this way. – Tao

Which is an entirely different problem... – Flaring

Answer 1

Simply creating this as a generic should work:

public void GetMinMaxRange<T>( DataTable data, string valueColumnName ) 
                                                             where T : IComparable<T>
{
   DataColumn column = data.Columns[valueColumnName];
   var min = data.AsEnumerable().Min(m => m.Field<T>(valueColumnName));
   var max = data.AsEnumerable().Max(m => m.Field<T>(valueColumnName));
}

Which you would then use as:

GetMinMaxRange<MyType>(dataTable, valueColumnName);

Bulbiferous

I think they're asking for runtime. This would still be compile time. – Printery

And you will need where T : IComparable<T> – Flaring

This answers my original question. I'll ask another one for the subtracting part. Thanks. – Tao

Answer 2

If you want to this on runtime, you can't use generics. You can rewrite your method like this:

public void GetMinMaxRangeTest(DataTable data, string valueColumnName)
{
    DataColumn column = data.Columns[valueColumnName];
    var min = data.AsEnumerable().Min(m => Convert.ChangeType(m[valueColumnName], column.DataType));
    var max = data.AsEnumerable().Max(m => Convert.ChangeType(m[valueColumnName], column.DataType));
}

I have tested this on a list, like this:

List<string> num = new List<string>() { 
    "1", "2", "3", "-1", "11", "10", "100"
};
var min = num.AsEnumerable().Min(m => Convert.ChangeType(m, typeof(int)));

and it yields the proper result: -1.

Frae

I get 'Object must implement IConvertible.' error for a DataTable rather than a List – Tao

What type does m[valueColumnName] have? double, decimal, int and string are all IConvertible so m[valueColumnName] having a value of this type can't be the problem – Frae

Answer 3

You can't do this so simple. column.DataType will be a SqlDbType and since it will only be known at runtime so the compiler can't verify that it is IComparable.

Flaring

column.DataType would return System.Type not SqlDbType according to my intellisense – Barbaric