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    <p>The problem is in <code>ParameterBuilder.SetConstant</code>, not <code>Activator.CreateInstance</code>. <code>SetConstant</code> is a function for defining the default value of an optional parameter, and requires a concrete value be provided as the constant. For ref classes, <code>null</code> is a valid concrete value, but for value classes prior to the creation of <code>Nullable&lt;&gt;</code>, <code>null</code> was not a valid value. How do you unbox <code>null</code> into an <code>int</code>, for example? So <code>SetConstant</code> checked value types to see if the concrete value passed in as the constant was <code>null</code> and throws an <code>ArgumentException</code> as a more descriptive error than the <code>NullReferenceException</code> you'd get for unboxing a null into a value class.</p> <p>In the case of a <code>Nullable&lt;&gt;</code>, <code>null</code> is now a valid concrete value for a value class. <code>Nullable&lt;&gt;</code> itself is a value class, as seen with <code>Activator.CreateInstance</code>, and unboxing <code>null</code> into a <code>Nullable&lt;int&gt;</code> has meaning. This is where <code>SetConstant</code> has a bug: it fails to take this into account and throws a 'descriptive' error for what isn't actually an error. In lieu of a bugfix from Microsoft, any call to <code>SetConstant</code> with a null <code>Nullable&lt;&gt;</code> will have to implement the behavior that's guarded by the incorrect conditional. This means digging into <code>ParameterBuilder</code>'s private methods and fields using reflection. Here's the code I made to handle <em>just</em> this case. The standard <code>SetConstant</code> function should be used in situations that don't manifest the bug.</p> <pre><code>//ModuleBuilder module : a parameter passed into the containing function, the module which is being built (which contains the method that has the optional parameter we are setting the constant for) //ParameterBuilder optParam : A parameter passed into the containing function, the parameter that is being built which is to have the null default value. MethodInfo method = typeof(TypeBuilder).GetMethods(BindingFlags.Static | BindingFlags.NonPublic) .Where(m =&gt; m.Name == "SetConstantValue" &amp;&amp; m.GetParameters().Length &gt; 0 &amp;&amp; m.GetParameters()[0].ParameterType.Name == "RuntimeModule") .Single(); var RuntimeHandle = typeof(ModuleBuilder).GetMethod("GetNativeHandle", BindingFlags.NonPublic | BindingFlags.Instance); method.Invoke(null, new object[] { RuntimeHandle.Invoke(module, new object[]{}), optParam.GetToken().Token, 0x12, null }); </code></pre> <p>I have reported the bug to Microsoft. They responded that it wouldn't be fixed in .NET 3.5, but it was added to the internal bug database.</p> <p><strong>UPDATE:</strong></p> <p>The bug has been fixed in .NET 4.0. <code>ParameterBuilder.SetConstant</code> now has a branch to the <code>constant == null</code> conditional that checks to see if a value type is a generic derived from <code>Nullable&lt;&gt;</code> and only throws the exception if it is not.</p>
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