C# - how to determine whether a Type is a number

Try this:

Type type = object.GetType();
bool isNumber = (type.IsPrimitiveImple && type != typeof(bool) && type != typeof(char));

The primitive types are Boolean, Byte, SByte, Int16, UInt16, Int32, UInt32, Int64, UInt64, Char, Double,and Single.

Taking Guillaume's solution a little further:

public static bool IsNumericType(this object o)
{   
  switch (Type.GetTypeCode(o.GetType()))
  {
    case TypeCode.Byte:
    case TypeCode.SByte:
    case TypeCode.UInt16:
    case TypeCode.UInt32:
    case TypeCode.UInt64:
    case TypeCode.Int16:
    case TypeCode.Int32:
    case TypeCode.Int64:
    case TypeCode.Decimal:
    case TypeCode.Double:
    case TypeCode.Single:
      return true;
    default:
      return false;
  }
}

Usage:

int i = 32;
i.IsNumericType(); // True

string s = "Hello World";
s.IsNumericType(); // False

Don't use a switch - just use a set:

HashSet<Type> NumericTypes = new HashSet<Type>
{
    typeof(decimal), typeof(byte), typeof(sbyte),
    typeof(short), typeof(ushort), ...
};

EDIT: One advantage of this over using a type code is that when new numeric types are introduced into .NET (e.g. BigInteger and Complex) it's easy to adjust - whereas those types won't get a type code.

None of the solutions takes Nullable into account.

I modified Jon Skeet's solution a bit:

    private static HashSet<Type> NumericTypes = new HashSet<Type>
    {
        typeof(int),
        typeof(uint),
        typeof(double),
        typeof(decimal),
        ...
    };

    internal static bool IsNumericType(Type type)
    {
        return NumericTypes.Contains(type) ||
               NumericTypes.Contains(Nullable.GetUnderlyingType(type));
    }

I know I could just add the nullables itself to my HashSet. But this solution avoid the danger of forgetting to add a specific Nullable to your list.

    private static HashSet<Type> NumericTypes = new HashSet<Type>
    {
        typeof(int),
        typeof(int?),
        ...
    };