Question

Possible Duplicates:
Nullable types and the ternary operator. Why won’t this work?
Conditional operator assignment with nullable<value> types?

This will not compile, stating “Type of conditional expression cannot be determined because there is no implicit conversion between ‘System.DateTime’ and ””

task.ActualEndDate = TextBoxActualEndDate.Text != "" ? DateTime.Parse(TextBoxActualEndDate.Text) : null;

This works just fine

 if (TextBoxActualEndDate.Text != "")
    task.ActualEndDate = DateTime.Parse(TextBoxActualEndDate.Text);
else
    task.ActualEndDate = null;

Answer 1

This doesn’t work because the compiler will not insert an implicit conversion on both sides at once, and null requires an implicit conversion to become a nullable type.

Instead, you can write

task.ActualEndDate = TextBoxActualEndDate.Text != "" ? 
    DateTime.Parse(TextBoxActualEndDate.Text) : new DateTime?();

This only requires one implicit conversion (DateTime to DateTime?).

Alternatively, you can cast either left side:

task.ActualEndDate = TextBoxActualEndDate.Text != "" ? 
    (DateTime?)DateTime.Parse(TextBoxActualEndDate.Text) : null;

This also requires only one implicit conversion.