当datetime为null时，如何将可空日期时间值转换为string.empty？

Question

I'm reading back a nullable DateTime? property then assigning that value to a string property in short date format.

I can convert the date time value to a short date string and assign to the IT_Date_String property. But I'm not sure how to assign a "" value to the string if the IT_Date is null.

How can you convert a datetime? value to string.empty when datetime? is null?

This is the assignment in linq:

var status_list = query_all.ToList().Select(r => new RelStatus
{
    IT_Date_String = r.IT_Date.Value.ToString("yyyy-MM-dd") != null ? r.IT_Date.Value : null
}).ToList();

And the properties in the model:

public DateTime? IT_Date { get; set; }
public string IT_Date_String { get; set; }

Answer 1

You're calling the IT_Date.Value.ToString(...) regardless of whether IT_Date actually has a value.

So you need to turn the expression around:

r.IT_Date.HasValue ? r.IT_Date.Value.ToString(...) : ""

This way ToString() will only be called when IT_Date has a value.

You can also implement this in the getter, as mentioned in a now-deleted comment:

public string IT_Date_String 
{ 
    get
    {
        return IT_Date.HasValue ? IT_Date.Value.ToString(...) : "";
    }
}

This way you won't have to reimplement the logic everywhere you access this model, and as a bonus, it will only be executed when it's actually requested.

There's also no need to explicitly use String.Empty , the string "" will be interned to the same at runtime .

Answer 2

In C# 6 you can do this:

IT_Date_String = r.IT_Date?.ToString("yyyy-MM-dd") ?? String.Empty;

The new ? checks if the thing on the left is null, if it is, the expression evaluates to null . If not, it just continues the evaluation.

Then, ?? checks whether the result of the first expression is null , which it would be if IT_Date is null. If it is, evaluate to String.Empty .

Answer 3

使用C＃6.0和null传播，您可以使用：

IT_Date_String = r.IT_Date?.ToString("yyyy-MM-dd") ?? String.Empty

Answer 4

这个适用于任何版本的框架：

IT_Date_String=string.Format("{0:yyyy-MM-dd}",IT_Date);