C＃Linq如何在我需要不为空的同一列中检查空值

Question

I have this query in linq

var finalResults = (from r in results.AsEnumerable()
                   where DateTime.Now.Subtract(r.Field<DateTime>("SentOn")).Minutes > 7
                   select r
                   ).ToList();

sometimes the SentOn column could be null and I want to ignore these cases. how can I remove these null cases ?

if it is required we can do a staging (another) table that has just the not null values and i can continue

the table is results so can I have another table, lets say results2 that doesn't have the null values or SentOn ?

Answer 1

If the field SentOn can be null , you have to return DateTime? instead of DateTime .

Try:

var finalResults = (from r in results.AsEnumerable()
                   where r.Field<DateTime?>("SentOn") != null &&
                         DateTime.Now.Subtract(r.Field<DateTime>("SentOn")).Minutes > 7
                   select r
                   ).ToList();

Alternatively, as pointed out in the comment, you can use DataRow.IsNull .

var finalResults = (from r in results.AsEnumerable()
                   where !r.IsNull("SentOn") &&
                         DateTime.Now.Subtract(r.Field<DateTime>("SentOn")).Minutes > 7
                   select r
                   ).ToList();

Answer 2

Just add the test in your WHERE clause:

var finalResults = (from r in results.AsEnumerable()
               where !r.IsNull("SentOn") 
                     && DateTime.Now.Subtract(
                           r.Field<DateTime>("SentOn")).Minutes > 7
               select r
               ).ToList();

Answer 3

You need to use the IsNull method to check if the value is null first. This can be done as follows:

var finalResults = (from r in results.AsEnumerable()
                   where !r.IsNull("SentOn") &&
DateTime.Now.Subtract(r.Field<DateTime>("SentOn")).Minutes > 7
                   select r
                   ).ToList();

Answer 4

You can check this in your .Where method with just 'if' statement:

var finalResults = results.AsEnumerable().Where(r =>
{
   if(r.Field<DateTime?>("SentOn") == null) return false;
   return DateTime.Now.Subtract(r.Field<DateTime?>("SentOn")).Minutes > 7;
}).ToList();

Answer 5

尝试这个

dt.AsEnumerable().Where(r => r.Field<DateTime?>("SentOn") != null && DateTime.Now.Subtract(r.Field<DateTime>("SentOn")).Minutes > 7);

Answer 6

A null check should suffice.

var finalResults = results.AsEnumerable().Select(r => !r.IsNull(("SentOn"))
            && DateTime.Now.Subtract(r.Field<DateTime>("SentOn")).Minutes > 7)

You could create an Extension Method if this is a repeated pattern.