为什么在IEnumerable上进行FirstOrDefault搜索 <Row> 基于RowIndex的在调试窗口中不起作用？

Question

The below line of code, which does a FirstOrDefault search on IEnumerable based on RowIndex doesn't work in debug windows (in Watch,Quick watch and Immediate window). It throws a System.NullReferenceException in these windows. I'm using Visual studio 2015 update 3.

sheetdata.Descendants<Row>().FirstOrDefault(p => p.RowIndex.Value == 2U)

But when I convert it to list and do the same search it works in these debug windows. Why is this discrepancy?

sheetdata.Descendants<Row>().ToList().FirstOrDefault(p => p.RowIndex.Value == 2U)

This discrepancy is not there when I run the code. I can see this discrepancy only when I try to debug the code in these debug windows.

Answer 1

I think the QueryProvider of OpenXml has a problem with handling of Convert.ToUInt32(2) , cause he needs to translate that into its own language (like a QueryProvider for SQL).

You should try to avoid converts in predicates and do them upfront, cause not all QueryProviders support all functionalities and might throw exceptions (mostly NotSupportedException ).

uint value = Convert.ToUInt32(2);
sheetdata.Descendants<Row>().FirstOrDefault(p => p.RowIndex.Value == value)

The call .ToList() will perform this in memory, cause it will read all values from your sheet and then finds the first matching value.