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    <p>This is a bug in .NET Framework 4 (and .NET 4.5, <strong>not in .NET 3.0 and .NET 3.5</strong>).</p> <p>Method <code>PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid(object item)</code> is causing the issue.</p> <p>Looked with .NET Reflector, its code looks like this:</p> <pre><code>private bool DetermineWhetherDBNullIsValid(object item) { PropertyInfo info; PropertyDescriptor descriptor; DependencyProperty property; DynamicPropertyAccessor accessor; this.SetPropertyInfo(this._arySVS[this.Length - 1].info, out info, out descriptor, out property, out accessor); string columnName = (descriptor != null) ? descriptor.Name : ((info != null) ? info.Name : null); object arg = ((columnName == "Item") &amp;&amp; (info != null)) ? this._arySVS[this.Length - 1].args[0] : null; return SystemDataHelper.DetermineWhetherDBNullIsValid(item, columnName, arg); } </code></pre> <p>Issue is in the following line:</p> <pre><code>object arg = ((columnName == "Item") &amp;&amp; (info != null)) ? this._arySVS[this.Length - 1].args[0] : null; </code></pre> <p>Code assumes that if <code>columnName</code> is <code>"Item"</code>, then property is indexer and tries to access its first argument via <code>args[0]</code> and this is where <code>NullReferenceException</code> occurs because <code>args</code> is <code>null</code> since property is not indexer. It just happens to be named <code>"Item"</code>.</p> <p>.NET implementers should have used <a href="http://msdn.microsoft.com/en-us/library/system.reflection.propertyinfo.getindexparameters.aspx" rel="noreferrer">PropertyInfo.GetIndexParameters()</a> on <code>info</code> and if returned array doesn't contain zero elements, make certain assumption that property is indexer. Or use <a href="http://msdn.microsoft.com/en-us/library/system.windows.data.binding.indexername.aspx" rel="noreferrer">Binding.IndexerName</a> for checking (Binding.IndexerName has value <code>"Item[]"</code>).</p> <p>Why issue arises only in Visual Studio debugger is much more subtle and it is hidden in following method: <strong>PresentationFramework.dll!MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid()</strong>.</p> <p>Here is a disassembled code:</p> <pre><code>private void DetermineWhetherDBNullIsValid() { bool flag = false; object item = this.GetItem(this.Length - 1); if ((item != null) &amp;&amp; AssemblyHelper.IsLoaded(UncommonAssembly.System_Data)) { flag = this.DetermineWhetherDBNullIsValid(item); } this._isDBNullValidForUpdate = new bool?(flag); } </code></pre> <p>Since <code>item</code> variable won't be null (it is actually an instance of <a href="http://msdn.microsoft.com/en-us/library/system.weakreference.aspx" rel="noreferrer">WeakReference</a> which holds <code>MainViewModel</code> instance), only condition for which failing method <code>DetermineWhetherDBNullIsValid(item)</code> is called is if System.Data.dll assembly is loaded, which is checked with <code>AssemblyHelper.IsLoaded(UncommonAssembly.System_Data)</code>.</p> <p>Visual Studio debugger will always load System.Data.dll because project is referencing it, although it is not using it. Outside Visual Studio debugger, System.Data.dll gets loaded only if used, which is never, and that is why application doesn't fail outside Visual Studio.</p> <p>You have following options to get rid of this issue:</p> <ol> <li>Rename property which is bound to <code>ComboBox.SelectedItem</code> to some name other than <code>"Item"</code> so that buggy .NET implementation doesn't assume anymore that property is indexer.</li> <li>Remove System.Data.dll from project references so it doesn't get loaded even in Visual Studio debugger.</li> </ol> <p>I find option 2 more fragile, since situation may arise that System.Data.dll must be loaded, either directly by your application or indirectly by some other loaded assembly.</p> <p>So I would go with option 1.</p>
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    1. COConfirming that both options are works as described - tested on VS2012 targeting .NET 4.5. Good job @Stipo and thanks for sharing! The moral of this story - **never** names any of your view-model's property as "**Item**" :)
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Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

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Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

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