c# struct fields are "never assigned to" warnings

Question

Based on http://alexreg.wordpress.com/2009/05/03/strongly-typed-csv-reader-in-c/, I created a DLL which can read different file types. I also have unit tests that run successfully. I create a struct and use it as the generic type.

Anyway, when I compile, I get a warning on each of the struct fields. For example: field 'FileReader.Tests.CsvReader.Record.Field1' is never assigned to, and will always have its default value 0

I am in fact setting the value with SetValueDirect() and when I run through the tests or debug the code, I can verify that. Why is it giving me that error then, and how can I avoid or fix it?

Here is some basic code to give you an idea. I'm guessing I haven't provided enough, but hopefully someone has a clue.

public abstract class FileReader<TRecord> : IDisposable where TRecord : struct
{
     public TRecord? ReadRecord()
     {
      List<string> fields;
      string rawData;

      this.recordNumber++;
      while (this.ReadRecord(this.fieldTypeInfoList.Length, out fields, out rawData))
      {
       try
       {
        // Insert the current record number to the beginning of the field list
        fields.Insert(0, this.recordNumber.ToString(CultureInfo.InvariantCulture));

        // Convert each field to its correct type and set the value
        TRecord record = new TRecord();
        FieldTypeInfo fieldTypeInfo;
        object fieldValue;

        // Loop through each field
        for (int i = 0; i < this.fieldTypeInfoList.Length; i++)
        {
         fieldTypeInfo = this.fieldTypeInfoList[i];

         bool allowNull = fieldTypeInfo.AllowNull == null ? this.AllowNull : fieldTypeInfo.AllowNull.Value;
         if (i >= fields.Count && !allowNull)
         {
          // There are no field values for the current field
          throw new ParseException("Field is missing", this.RecordNumber, fieldTypeInfo, rawData);
         }
         else
         {
          // Trim the field value
          bool trimSpaces = fieldTypeInfo.TrimSpaces == null ? this.TrimSpaces : fieldTypeInfo.TrimSpaces.Value;
          if (trimSpaces)
          {
           fields[i] = fields[i].Trim();
          }

          if (fields[i].Length == 0 && !allowNull)
          {
           throw new ParseException("Field is null", this.RecordNumber, fieldTypeInfo, rawData);
          }

          try
          {
           fieldValue = fieldTypeInfo.TypeConverter.ConvertFromString(fields[i]);
          }
          catch (Exception ex)
          {
           throw new ParseException("Could not convert field value", ex, this.RecordNumber, fieldTypeInfo, rawData);
          }

          fieldTypeInfo.FieldInfo.SetValueDirect(__makeref(record), fieldValue);
         }
        }

        return record;
       }
       catch (ParseException ex)
       {
        ParseErrorAction action = (ex.FieldTypeInfo.ParseError == null) ? DefaultParseErrorAction : ex.FieldTypeInfo.ParseError.Value;

        switch (action)
        {
         case ParseErrorAction.SkipRecord:
          continue;

         case ParseErrorAction.ThrowException:
          throw;

         case ParseErrorAction.RaiseEvent:
          throw new NotImplementedException("Events are not yet available", ex);

         default:
          throw new NotImplementedException("Unknown ParseErrorAction", ex);
        }
       }
      }

      return null;
     }
}

Answer 1

It seems that the compiler is not capable of detecting such "indirect" assignments. It can only detect direct assignments like field=value.

You can anyway disable specific compiler warnings. Assuming that you are using Visual Studio, see here: http://msdn.microsoft.com/en-us/library/edzzzth4.aspx

Answer 2

The compiler is never going to be able to spot reflection. By definition, by using reflection you have stepped outside the compiler.

IMO, though, this is a bad use of structs - that looks very much like it should be working on classes...

Answer 3

I hate when I reinvent the wheel. FileHelpers from http://www.filehelpers.com/ already does this in a very similar way and of course covers more edge cases. They have you define a class with attributes instead of a struct (as Marc suggested). If you set their record definition as non-public, you get the same compiler warnings I was getting.

Answer 4

If using struct instead of class you should know why you do it.

Some (rare) cases where you should use struct:

1. P/Invoke (when the native function uses "structs") - this is (imho) one of the the reason why MS added struct to .NET (in order to be compatible with native code)
2. structs are so called value types. structs should be very small (e.g. struct DateTime, struct GpsCoordinates are reasons using struct instead of classes since struct are "faster" than classes because the GC need not to care about it
3. If you don't know what to use: use class!!

--hfrmobile