Check if read() has been called on SqlDataReader

Question

I'm writing some helper functions that take a SqlDataReader and process the results. Now normally you'd structure the loop like so:

while(reader.read()){
    // Read fields
}

But I my be passing the function a reader which is already positioned on a row, and in this case I want it to process the current row. The code above will only output the entire result set if read() had not been called prior to invoking the function.

I want the function to do something like this:

if(!reader._dataReady)
    rowExists = reader.read() // Read 
while(rowExists){
    // read fields
    rowExists = reader.read()
}

_dataReady is an actual member of SqlDataReader and seems to do what I need, however it's private! Surely there's a way to determine if the reader is positioned on an actual row, short of trying to read the field and catching the exception.

Can't you just force your users to always do a read before calling your helper? Failure to do so would lead to an exception which you could catch, wrap, and rethrow. Your target audience is obviously developers, so force them to follow your API. — M.Babcock
– M.Babcock, Commented Feb 22, 2012 at 1:23

b0rg · Accepted Answer · 2012-02-22 15:13:16Z

2

why not just subclass SqlDataReader, and create Read() method? I have it in all my projects, now:

public class SafeDataReader : IDataReader, SqlDataReader
{
    public IDataReader reader {get;set;}
    public int GetInt32(string aFieldName, int aDefault);
    public int? GetInt32Nullable(string aFieldName);
    .... 
}

answered Feb 22, 2012 at 15:13

b0rg

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1 Comment

Trent Over a year ago

Good idea. I could store an enum that tracked the reader position: - SafeDataReaderPosition.BeforeRows - SafeDataReaderPosition.OnRow - SafeDataReaderPosition.EndOfFile. All without having to sacrifice async performance by using a dataset. Don't know why this isn't part of the implementation already...

Chris Shain · Accepted Answer · 2012-02-21 23:53:10Z

1

You can use the FieldCount property. As per the documentation, it returns zero if not positioned in a valid recordset.

Some other solutions:

Do your Read() looping entirely outside of your helper method (and always operate only on the current row)
Do your Read() looping entirely within the helper method, and always pass it fresh data readers that have not been advanced
Do your first Read() outside of the data reader, continue Read() looping within the helper method, and always assume that the data reader has been advanced to the first record.
Probably overkill: write a wrapping implementation of IDataReader that forwards all calls to the underlying data reader (probably passed in via the constructor), but exposes a property indicating whether Read has ever been called.

answered Feb 21, 2012 at 23:53

Chris Shain

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2 Comments

Trent Over a year ago

Just tested, and the fieldCount property does not return zero if read() hasn't been called yet. It only returns zero if SQL didn't return a result set at all.

mcw Over a year ago

To clarify: Trent is correct - if the reader has been populated, but Read() has not been called, and the reader has rows with fields, then FieldCount will be set to the number of fields in the result set.

IvanH · Accepted Answer · 2018-04-06 15:24:06Z

1

Reflection is dangerous but often useful. Here is an extension

public static bool? DataReady(this SqlDataReader rd) {
            var sharedState = rd.GetType()?.GetField("_sharedState", 
                 BindingFlags.NonPublic | BindingFlags.Instance)?.GetValue(rd);
            return (bool?)sharedState?.GetType().GetField("_dataReady",
                BindingFlags.NonPublic | BindingFlags.Instance)?.GetValue(sharedState);
 }

Information taken from SqlDataReader referencesource.

answered Apr 6, 2018 at 15:24

IvanH

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Comments

Steve Czetty · Accepted Answer · 2012-02-21 23:55:59Z

0

It is sort of overloading the purpose, but the following should work:

rowExists = reader.FieldCount > 0;

I suspect that you are going to find yourself having problems with a design that has you doing Read()s in different locations in your code. Try doing only data manipulation in your method (and passing it as an IDataRecord instead of an IDataReader), and leave the record traversal logic to the caller.

answered Feb 21, 2012 at 23:55

Steve Czetty

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