Using .Net, how can I determine if a type is a Numeric ValueType?

You can find out if a variable is numeric using the Type.GetTypeCode() method:

TypeCode typeCode = Type.GetTypeCode(desiredType);

if (typeCode == TypeCode.Double || typeCode == TypeCode.Integer || ...)
     return true;

You'll need to complete all the available numeric types in the "..." part ;)

More details here: TypeCode Enumeration

Great article here Exploring IsNumeric for C#.

Option 1:

Reference Microsoft.VisualBasic.dll, then do the following:

if (Microsoft.VisualBasic.Information.IsNumeric("5"))
{
 //Do Something
}

Option 2:

public static bool Isumeric (object Expression)
{
    bool f;
    ufloat64 a;
    long l;

    IConvertible iConvertible = null;
    if ( ((Expression is IConvertible)))
    {
       iConvertible = (IConvertible) Expression;
    }

    if (iConvertible == null)
{
   if ( ((Expression is char[])))
   {
       Expression = new String ((char[]) Expression);
       goto IL_002d; // hopefully inserted by optimizer
   }
   return 0;
}
IL_002d:
TypeCode typeCode = iConvertible.GetTypeCode ();
if ((typeCode == 18) || (typeCode == 4))
{
    string str = iConvertible.ToString (null);
   try
   {
        if ( (StringType.IsHexOrOctValue (str, l)))
   {
        f = true;
        return f;
   }
}
catch (Exception )
{
    f = false;
    return f;
};
return DoubleType.TryParse (str, a);
}
return Utils.IsNumericTypeCode (typeCode);
}

internal static bool IsNumericType (Type typ)
{
bool f;
TypeCode typeCode;
if ( (typ.IsArray))
{
    return 0;
}
switch (Type.GetTypeCode (typ))
{
case 3: 
case 6: 
case 7: 
case 9: 
case 11: 
case 13: 
case 14: 
case 15: 
   return 1;
};
return 0;
}

''// Return true if a type is a numeric type.
Private Function IsNumericType(ByVal this As Type) As Boolean
    ''// All the numeric types have bits 11xx set whereas non numeric do not.
    ''// That is if you include char type which is 4(decimal) = 100(binary).
    If this.IsArray Then Return False
    If (Type.GetTypeCode(this) And &HC) > 0 Then Return True
    Return False
End Function

You can now use the .NET Framework method

typeof(decimal?).IsNumericType()

Use Type.IsValueType() and TryParse():

public bool IsInteger(Type t)
{
   int i;
   return t.IsValueType && int.TryParse(t.ToString(), out i);
}