How to use Field 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;
    }

Exactly _why_ do you need it? Try to finish that method, you are not using min/max yet. — Henk Holterman, Sep 12 '11 at 16:20
You can write that last sample as `public static T GetMinMaxRange(...) ...` which indeed makes sense but needs to spec T at compiletime. — Henk Holterman, Sep 12 '11 at 16:28
@Henk - yes, but I can't seem to subtract min from max in this way. — O.O, Sep 12 '11 at 16:35

score 6 · Accepted Answer · answered Sep 12 '11 at 16:13

6

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);

answered Sep 12 '11 at 16:13

Jamie Dixon

49,653
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1

I think they're asking for runtime. This would still be compile time. – Paul Phillips Sep 12 '11 at 16:15
2

And you will need `where T : IComparable` – Henk Holterman Sep 12 '11 at 16:16
This answers my original question. I'll ask another one for the subtracting part. Thanks. – O.O Sep 12 '11 at 17:02

score 2 · Answer 2 · answered Sep 12 '11 at 16:31

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.

I get 'Object must implement IConvertible.' error for a DataTable rather than a List — O.O, Sep 12 '11 at 16:54
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 — AlexB, Sep 12 '11 at 17:09

score 1 · Answer 3 · answered Sep 12 '11 at 16:15

1

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.

answered Sep 12 '11 at 16:15

Henk Holterman

236,989
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`column.DataType` would return `System.Type` not `SqlDbType` according to my intellisense – Stuart Blackler Sep 12 '11 at 16:21

How to use Field with Type?

3 Answers3