试图合成DateTime？ 变量导致“方法ToString没有重载需要1个参数错误”

Question

I am building out a ViewModel and in it I am trying to format a DateTime in MMMM dd, yyyy format but it is throwing the error

No overload for method 'ToString' takes 1 argument

I used http://msdn.microsoft.com/en-us/library/8kb3ddd4(v=vs.110).aspx to come up with the code

DateUpdated.ToString("MMMM dd, yyyy")

but that is apparently wrong. What is the correct way to format the date?

ViewModel:

public class ConversionFactorsVM
{
    [Required]
    public int TankID { get; set; }

    [ReadOnly(true), DisplayName("Product Name")]
    public string ProductName { get; set; }

    [ReadOnly(true), DisplayName("Product ID")]
    public int Productnumber { get; set; }

    [Required, Range(0, 200.9, ErrorMessage = "Gravity must be between 0 and 200.9")]
    public decimal Gravity { get; set; }

    [Required, Range(0, 200.9, ErrorMessage = "Temperature must be between 0 and 200.9")]
    public decimal Temperature { get; set; }

    [ReadOnly(true)]
    public decimal Factor { get; set; }

    public DateTime? DateUpdated { get; set; }

    [DisplayName("Last Updated")]
    public string LastUpdate
    {
        get
        {
            if (DateUpdated.HasValue)
            {
                return DateUpdated.ToString("MMMM dd, yyyy");
            }
            else
            {
                return "Never Updated.";
            }
        }
    }
}

Answer 1

您必须使用Nullable<DateTime>.Value因为那是DateTime ：

return DateUpdated.Value.ToString("MMMM dd, yyyy");

Answer 2

You should use Value propery for getting the value of Nullable<DateTime>

Gets the value of the current Nullable<T> object if it has been assigned a valid underlying value.

Here an example in LINQPad ;

DateTime? DateUpdated = DateTime.Now;
DateUpdated.Value.ToString("MMMM dd, yyyy").Dump();

Output will be;

January 14, 2014