Nullable datetime conversion error

Question

I am reading field values which may have an unformatted date in them or whitespace, i need to convert the string in the filled fields to datetime and write it back. I'm trying the following but get the error "Nullable object must have a value":

DateTime? localVersion = null;
DateTime? serverVersion = null;

if(!string.IsNullOrWhiteSpace(item.cellValueLocal))
{
    localVersion = DateTime.ParseExact(item.cellValueLocal, "ddMMyyyy", System.Globalization.CultureInfo.InvariantCulture);
}

if (!string.IsNullOrWhiteSpace(item.cellValueServer))
{
    serverVersion = DateTime.ParseExact(item.cellValueServer, "ddMMyyyy", System.Globalization.CultureInfo.InvariantCulture);
}

localVersion.Value.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture);
serverVersion.Value.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture);

Can anyone offer any guidance on what the issue might be here please?

Try to set them to a default value such as DateTime? localVersion = new DateTime(2015, 1, 18);' — Majestic
– Majestic, Commented Jun 21, 2016 at 14:01
What do you expect to happen if (say) item.cellValueLocal is null or whitespace? — Jon Skeet
– Jon Skeet, Commented Jun 21, 2016 at 14:01
Think about what @JonSkeet is saying. If the value is a string or whitespace, localVersion and serverVersion won't have a value you can call toString() on. — Evan Trimboli
– Evan Trimboli, Commented Jun 21, 2016 at 14:04
Additionally, what do you expect the last lines to achieve when they do work? You're calling ToString(), but not doing anything with the return value. — Jon Skeet
– Jon Skeet, Commented Jun 21, 2016 at 14:04

knittl · Accepted Answer · 2016-06-21 14:03:28Z

3

If your cellValue is null (or whitespace), your DateTime instances will not be initialized and still be set to null. You cannot access .Value on a nullable instance without a value. You have to check with .HasValue first:

if (localVersion.HasValue) {
  localVersion.Value.ToString(...);
}

answered Jun 21, 2016 at 14:03

knittl

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2 Comments

Chris Dunaway Over a year ago

This won't actually do anything since you're not assigning the result of the ToString call to a variable.

knittl Over a year ago

@ChrisDunaway: yes. Execute additional code as needed …

theB · Accepted Answer · 2016-06-21 14:16:18Z

1

The lines:

localVersion.Value.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture);
serverVersion.Value.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture);

Are errors. If item.cellValueLocal or item.cellValueServer is null or empty then localVersion.Value and serverVersion.Value will never be assigned a non-null value, and therefore Value will throw an exception when you access it.

You need to replace the operations with:

if (localVersion.HasValue)
    localVersion.Value.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture);

Alternatively you could use the null coalescing operator:

DateTime version = localVersion ?? DateTime.Now;

Replacing DateTime.Now with an appropriate default value. In C# 6 you can use the ?. syntax to simplify it further to:

string version = localVersion?.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture) ?? String.Empty;

Which does the null check inline.

edited Jun 21, 2016 at 14:16

answered Jun 21, 2016 at 14:05

theB

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2 Comments

juharr Over a year ago

Actually Value isn't null in that case it's just that the Nullable<T> type guards access to Value based on HasValue being false.

theB Over a year ago

@juharr - Fair enough, I changed the wording.

Mike Miller · Accepted Answer · 2016-06-21 14:26:29Z

0

If you are compiling the latest version of c# (6.0) compiler then you could shorten this further to be

localVersion?.ToString("dd.MM.yyyy", CultureInfo.InvariantCulture)

edited Jun 21, 2016 at 14:26

answered Jun 21, 2016 at 14:11

Mike Miller

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1 Comment

Chris Dunaway Over a year ago

This will do nothing as you're not assigning the result of the ToString call to a variable.

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