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POIronpython: Debugging a null reference exception
 

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    <p>I have previously asked <a href="https://stackoverflow.com/questions/6712051/ironpython-function-works-in-cpython-mysterious-null-pointer-exception-in-ironp">this</a> question when I was seeing a null pointer exception. In that case it turned out that what I was seeing was in fact a bug in IronPython. </p> <p>Now I recently came across this error again when using a third party library (written in C#). The library is the <a href="http://dotspatial.codeplex.com/" rel="nofollow noreferrer">DotSpatial</a> library and it turns out that I accidentally created a condition in my IronPython code that led to a side effect in the DotSpatial method breaking. For reference the call and error message are:</p> <pre><code>&gt;&gt;&gt; myScheme.CreateCategories(myLayer.DataSet.DataTable) Traceback (most recent call last): File "&lt;stdin&gt;", line 1, in &lt;module&gt; SystemError: Object reference not set to an instance of an object. </code></pre> <p>This was particularly difficult to debug because the problem was not with the myScheme object or the DataTable I was passing to its method, but with another property of myScheme that I had set incorrectly, which I didn't even know that CreateCategories accessed.</p> <p>To figure out what was going wrong here I had to read through the DotSpatial <a href="http://dotspatial.codeplex.com/SourceControl/changeset/view/3f292a701a9d#DotSpatial.Symbology/DotSpatial.Symbology/FeatureScheme.cs" rel="nofollow noreferrer">source code</a>. While I don't have too much trouble doing that, I was wondering if there are any easier ways to debug such an error? Or am I stuck with bad error messages because I'm working with IronPython and a third party library?</p>
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