GetValue，如何将空值处理为Linq语句

Question

I'm having problems exporting an entity to excel.

I have this Linq:

List<PropertyInfo> props = FieldsToExport(EntitySet.AsQueryable());
int iCols = props.Count();

foreach (var Entity in EntitySet)
{
    var values = Enumerable.Range(0, iCols)
        .Select(ii => props[ii].GetValue(Entity)
                                .HandleNullProps()
        );

    // .... write in the sheet
} 

My problem is when the [ii] property of my entity has a null value, it is throwing a null exception even if HandleNullProps() is filtering it..

Here is my HandleNullProps()

public static object HandleNullProps(this object s)
{
    if (s == null)
        return string.Empty;
    else
        return s;
    }
}

Answer 1

if props[ii] is null, then calling GetValue(Entity) will cause a NullReferenceException

your handleNullProps method is not even reached - the exception is thrown before that

update your select to something like this

var values = Enumerable.Range(0, iCols).Select(ii => props[ii] != null ? props[ii].GetValue(Entity) : String.Empty)

and now your handleNullProps method becomes unnecessary.

Edit

Since you're trying Extension Methods, you could use them to get your values anyway like this

var values = Enumerable.Range(0, iCols).Select(ii => props[ii].GetValueOrDefault(Entity))

and let the method be defined like this

public static object GetValueOrDefault(this object s, object entity)
{
    if (s == null)
        return string.Empty;
    else
        return s.GetValue(entity);
    }
}

keep in mind that I put object to both parameters because I don't know they proper types - set the correct types to both parameters and should work fine

the OrDefault part is inspired in one of the linq methods FirstOrDefault

Answer 2

It's not clear what is null in your case.

If it's the Entity then you should filter it out before:

foreach (var Entity in EntitySet.Where(e=>e != null))

If it's the props[ii] then .Select(ii => props[ii]?.GetValue(Entity) should do it.
And You can filter them out after wards the same way as before.
Your code can become:

foreach (var Entity in EntitySet.Where(e => e != null))
{
    var values = Enumerable.Range(0, iCols)
        .Select(ii => props[ii]?.GetValue(Entity)).Where(v => v !=  null);

    // .... write in the sheet
} 

Update: if it's props[ii] then i suggest that you filter it beofre:

List<PropertyInfo> props = FieldsToExport(EntitySet.AsQueryable()).Where(p=>p!=null).ToList();

And the whole code can become simpler anyway:

var values = props.Select(p=>p.GetValue(Entity)).Where(v => v !=  null);