Invalid Cast from 'System.Int32' to 'System.Nullable'1[[System.Int32, Mscorlib]]

Invalid cast from 'System.Int32' to 'System.Nullable`1[[System.Int32, mscorlib]]

You have to use Nullable.GetUnderlyingType to get underlying type of Nullable.

This is the method I use to overcome limitation of ChangeType for Nullable

public static T ChangeType<T>(object value) 
{
   var t = typeof(T);

   if (t.IsGenericType && t.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) 
   {
       if (value == null) 
       { 
           return default(T); 
       }

       t = Nullable.GetUnderlyingType(t);
   }

   return (T)Convert.ChangeType(value, t);
}

non generic method:

public static object ChangeType(object value, Type conversion) 
{
   var t = conversion;

   if (t.IsGenericType && t.GetGenericTypeDefinition().Equals(typeof(Nullable<>))) 
   {
       if (value == null) 
       { 
           return null; 
       }

       t = Nullable.GetUnderlyingType(t);
   }

   return Convert.ChangeType(value, t);
}

Invalid cast from System.Int32 to Nullable in case of Enum properties

Because you're dealing only with Enums, you may simply change your code to this:

var safeValue = Enum.ToObject(t, dat[i]);

Invalid cast from 'System.Double' to 'System.Nullable`

Simply : that isn't a supported usage of Convert.ChangeType. You need to give it the non-nullable form, i.e. double. You could use Nullable.GetUnderlyingType:

return value == DBNull.Value ? default(T) : (T)Convert.ChangeType(value,
      Nullable.GetUnderlyingType(typeof(T)) ?? typeof(T));

but this will have overhead; personally, I suspect it might be easier to use a Nullable<T> specific method:

public static T? GetValue<T>(object value) where T : struct
{
    if(value == null || value is DBNull) return null;
    if(value is T) return (T)value;
    return (T)Convert.ChangeType(value, typeof(T));
}

Or even better yet: use a an ORM or micro-ORM, so you don't need to do this stuff (also: they'll do a better job of it, by thinking about the type conversions during the meta-programming code, rather than the per-value execution code).

How to Convert a String to a Nullable Type Which is Determined at Runtime?

There are two problems.

Firstly, Convert.ChangeType just plain does not support nullable types.

Secondly, even if it did, by boxing the result (assigning it to an object), you'd already be converting it to a DateTime.

You could special case nullable types:

string s = "2012-02-23 10:00:00";
Type t = Type.GetType("System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]");
object d;

if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
{
    if (String.IsNullOrEmpty(s))
        d = null;
    else
        d = Convert.ChangeType(s, t.GetGenericArguments()[0]);
}
else
{
    d = Convert.ChangeType(s, t);
}

InvalidCastException: Unable to cast object of type 'System.DBNull' to type 'System.Nullable`1[System.Int32]'

From DBNull Documentation:

DBNull represents a nonexistent value, where null denotes absence of
reference to the object.

To fix your issue,

Null check before changing value's type and then assign it to the variable

result.Add (
     new GetActiveUserPackagesForOpenBillingPeriodResult {
     Amount = (decimal) reader["Amount"],
     AcceptanceActID =  Convert.IsDBNull(reader["AcceptanceActID"]) ? null : (int?) reader["AcceptanceActID"],
     PackageID = (int) reader["PackageID"],
     UserID = (int) reader["UserID"],
     AccountID = (int) reader["AccountID"],
     HasChangedPackage = (bool) reader["HasChangedPackage"],
        });

C# - Problems casting from System.String to System.NullableSytem.Int32 in bindings

tried to play around with this, and here's how I solved it - when creating the binding on the text box:

this.text_box.DataBindings.Add(new CustomBinding("Text", this.test, "some_int", false, DataSourceUpdateMode.OnPropertyChanged));

set the formatting flag to "true" (it's the third parameter).

once you do that, you don't need the custom binding anymore...I commented the code in OnParse out and it works. I use just a Binding object instead of CustomBinding, and it still works :)

check this blog for some details:
http://blog.jezhumble.net/?p=3

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