There is no implicit conversion between null and datetime

Question

Following code read a piece data from given DataRow(modelValue) and parse it to a nullable DateTime instance.

Question: Please see the code sections under L1 & L2 where both are technically equal (If i am not making any schoolboy error). However, L1 works as expected but not L2. I am getting

there is no implicit conversion between null and datetime

when I execute the code under L2. Can someone advise me ?

        DateTime? CallBack;

        var callBackDate = modelValue["CallBack"] == DBNull.Value ? null : modelValue["CallBack"].ToString();
        //Parsing
        DateTime cdate;
        if (!DateTime.TryParse(callBackDate, out cdate))
            cdate = DateTime.MinValue;


        //L1
        if (cdate==DateTime.MinValue)
            CallBack = null;
        else
           CallBack = cdate.Date;

       //L2  
       CallBack = cdate == DateTime.MinValue?null:cdate.Date;

This has been asked before a lot, for example [here](http://stackoverflow.com/questions/75746/conditional-operator-assignment-with-nullablevalue-types). IMO, the cleanest workaround that still uses `?:` is `... ? default(DateTime?) : ...`. — , Dec 05 '13 at 11:29
The compiler fails to infer that the expression `cdate == DateTime.MinValue?null:cdate.Date` should return `DateTime?` instead of `DateTime` (because `cdate.Date` is `DateTime`). So, it tries to convert null to `DateTime`... and fails. — Theraot, Dec 05 '13 at 11:42

score 40 · Answer 1 · answered Dec 05 '13 at 11:29

40

You need to tell the compiler that the null should be treated as DateTime?. Otherwise the compiler doesn't know what type null is.

CallBack = cdate == DateTime.MinValue ? (DateTime?)null : cdate.Date;

answered Dec 05 '13 at 11:29

Jakub Konecki

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"Otherwise the compiler doesn't know what type null is." That's not the problem here. The same expression would have worked if the third operand was a reference type or a nullable type, instead of a non-nullable value type. The problem is that there is no implicit conversion between null and `DateTime`. – dcastro Dec 05 '13 at 11:37
@Jakub Konecki, Please see the code sections L1 & L2 where both are technically equal and i presume csc.exe use same style of compilation for those two sections. Also, if i go with your result then how DateTime? dt=null works fine (whether you declare it within a method or as a field) ? We don't do any explicit casting here right. – S.N Dec 05 '13 at 11:41
@Nair the compiler is trying to cast `null` to `DateTime`, not to `DateTime?`. Yes it can cast `null` to `DateTime?`. – Theraot Dec 05 '13 at 11:44
@Nair see my answer, where I explain why the compiler is not able to evaluate the expression. – dcastro Dec 05 '13 at 11:48

dcastro · Accepted Answer · 2013-12-05T13:02:43.630

(Z) ? X : Y

The ternary operator requires that an implicit conversion exists from the second operand (X) to the third operand (Y), or from Y to X.

Since null cannot be implicitly converted to DateTime, nor DateTime to null, the expression cannot be evaluated. More on this: Type inference woes by Eric Lippert.

You have to cast null to DateTime?. By doing so, X will be of type DateTime? and Y will be of type DateTime. Since there is an implicit conversion from DateTime to DateTime?, the expression can be evaluated, and it will return a value of type DateTime?.

Alternatively, and following the same logic, you could also cast the third operand Y to DateTime?.

Tim Schmelter · Answer 3 · 2013-12-05T11:38:14.383

5

Why don't you use the DataRow.Field extension method which supports nullable types in the first place?

DateTime? CallBack = modelValue.Field<DateTime?>("CallBack");

But since you actually have a string column you need to parse it first:

DateTime? CallBack = null;
string callBackDate = modelValue.Field<string>("CallBack");
if(!string.IsNullOrWhiteSpace(callBackDate))
{
    DateTime cdate;
    if(DateTime.TryParse(callBackDate, out cdate))
        CallBack = cdate;
}

That's all.

edited Dec 05 '13 at 11:38

answered Dec 05 '13 at 11:30

Tim Schmelter

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It is handy though I it differ from my question. Thanks – S.N Dec 05 '13 at 11:34
@Nair: I have edited my answer to proide the correct parsing. – Tim Schmelter Dec 05 '13 at 11:38

score 1 · Answer 4 · answered Dec 05 '13 at 11:39

1

You could do the following:

        DateTime aDateTime = DateTime.MinValue;
        DateTime? aNullableDateTime = aDateTime == DateTime.MinValue ? null : new DateTime?(aDateTime.Date);

which uses the Nullable(Of T) structure.

answered Dec 05 '13 at 11:39

lex87

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There is no implicit conversion between null and datetime

4 Answers4