DateTime 01/01/0001

and your question is?

i do
public DateTime? fec_obs_concal { get; set; }

i have this error:
CS0266 C# Cannot implicitly convert type 'System.DateTime?' to 'System.DateTime'. An explicit conversion exists (are you missing a cast?)

have you tried Shaun comment?

Did you also change the definition of concal_fecha to be a Nullable DateTime?

You will need to look through your code for other places where dates will need to allow nulls and make the needed changes.

thanks

I made the change throughout the project

Date's cannot be empty.  The default value (which equates to the DateTime.Min value) is '01/01/0001 12:00 AM'.  Proof of concept -

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace EE_Q28962384
{
	class Program
	{
		static DateTime date1;

		static void Main(string[] args)
		{
			Console.WriteLine("Date1 = {0}", date1);
			Console.WriteLine("Default DateTime = {0}", default(DateTime));
			Console.WriteLine("DateTime.Min = {0}", DateTime.MinValue);
			Console.ReadLine();
		}
	}
}

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Produces the following output -If you try to parse an empty string you end up with -Which means you are left with using a try...catch or DateTime.TryParse.  If the parse fails, the output is still the MinValue.  Proof of concept:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace EE_Q28962384
{
	class Program
	{
		static DateTime date1;

		static void Main(string[] args)
		{
			Console.WriteLine("Date1 = {0}", date1);
			Console.WriteLine("Default DateTime = {0}", default(DateTime));
			Console.WriteLine("DateTime.Min = {0}", DateTime.MinValue);
			if (DateTime.TryParse("", out date1))
				Console.WriteLine("Date1 has changed successfully - {0}", date1);
			else
				Console.WriteLine("Date1 failed to changed, MinValue assigned - {0}", date1);
			Console.ReadLine();
		}
	}
}

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Which produces the following output -
Just an FYI, trying to parse a null value results in the MinValue being set for the date.

That being said, as the others have stated you can add Null support to the DateTime variable by specifying a Nullable<DateTime> or DateTime?.  This will allow for you to support null values for Dates, but remember, that DateTime's cannot be empty.

-saige-