Can not convert type object to type long

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Day 3 trying to solve this.

I'm trying to set a variable in C# equal to the values I retrieve from the database, as I have done hundreds of times. I have literally no idea what I'm doing wrong.

My problem is with DBToType<t> while iterating DataTable.Rows to get a field from it's ItemArray.

for ( int i = 0 ; i < dt.Rows.Count - 1 ; i++ ) //Looping through rows
{
	long COLUMN_PROPID = DBToType<long>( dt.Rows[i].ItemArray[5] ); 
}

DBToType produces "Unable to convert from type object to type long"

/// <summary>
		/// Converts DB types to C# Types and ensures that DBNull gets converted to the default value for the type.
		/// </summary>
		/// <typeparam name="T">The expected return type</typeparam>
		/// <param name="value">The value of a column, straight form the database provider</param>
		/// <returns>The value as its coorisponding C# Type, with DBNull converted to the default value of the type. </returns>
		public T DBToType<T> ( T value )
		{
			if ( !Convert.IsDBNull ( value ) )
			{
				return ( T ) Convert.ChangeType ( value , typeof ( T ) );
			}
			else
			{
				return default ( T );   //returns the default for the type... not null
			}
		}

What I have tried:

Everything under the sun...

This was my first attempt at solving this with a generic method.

Posted 26-Dec-17 12:38pm

User98743

Updated 26-Dec-17 14:55pm

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BillWoodruff · Answer 1 · 2017-12-26T14:55:00

To exclude the possibility that you are getting an object that can't be converted to a long, test with another function that uses Int64.TryGetValue, and break on error to examine the data.

I would rather use hard-coded Type specific conversion methods, which throw useful errors. Use of 'ChangeType is expensive. Here's an example of the type of Extension method I use:

public static class ConversionExtensions
{
    public static Int32 ToInt(this string str, bool doThrowOnError = true, int defaultValue = Int32.MinValue)
    {
        int i;

        if (Int32.TryParse(str, out i))
        {
            return i;
        }

        if (doThrowOnError)
        {
            throw new ArgumentException($"{str} is not convertible to Int32");
        }

        return defaultValue;
    }
}

If you want to "go generic," then try something like this:

using System.ComponentModel; // required

// written quickly: not fully tested
public T2 DbTypeConverter<T1,T2>(T1 value)
{
    if (value == null || Convert.IsDBNull(value))
    {
        return default(T2);
    }

    TypeConverter typeConverter = TypeDescriptor.GetConverter(typeof(T2));

    if (typeConverter.CanConvertFrom(typeof(T1)))
    {
        try
        {
            return (T2) Convert.ChangeType(value, typeof(T2));
        }
        catch (Exception ex)
        {
            switch (ex.GetType().Name)
            {
                // fill these in with meaningful actions !
                case "InvalidFormatException":
                    break;
                case "InvalidCastException":
                    break;
                case "FormatException":
                    break;
                case "OverflowException":
                    break;
                case "ArgumentException":
                    break;
            }
        }
    }

    return default(T2); //returns the default for the type... not null
}

// simple tests
    // long lng1 = DbTypeConverter<string, long>("57657777");
    
    // long lng2 = DbTypeConverter<string, long>("34.45");
    
    // long lng3 = DbTypeConverter<string, long>(DateTime.Now.ToString());

Good info on what you can, and can't, do with TypeConverter: [^]