virtualCERCall MDA

Artikkeli
06/04/2024

Note

This article is specific to .NET Framework. It doesn't apply to newer implementations of .NET, including .NET 6 and later versions.

The virtualCERCall managed debugging assistant (MDA) is activated as a warning indicating that a call site within a constrained execution region (CER) call graph refers to a virtual target, that is, a virtual call to a non-final virtual method or a call using an interface. The common language runtime (CLR) cannot predict the destination method of these calls from the intermediate language and metadata analysis alone. As a result, the call tree cannot be prepared as part of the CER graph and thread aborts in that subtree cannot be automatically blocked. This MDA warns of cases where a CER might need to be extended by using explicit calls to the PrepareMethod method once the additional information required to compute the call target is known at run time.

Symptoms

CERs that do not run when a thread is aborted or an application domain is unloaded.

Cause

A CER contains a call to a virtual method that cannot be prepared automatically.

Resolution

Call PrepareMethod for the virtual method.

Effect on the Runtime

This MDA has no effect on the CLR.

Output

Method 'MethodWithCer', while executing within a constrained execution region, makes a call
at IL offset 0x0024 to 'VirtualMethod', which is virtual and cannot be prepared automatically
at compile time. The caller must ensure this method is prepared explicitly at
runtime before entering the constrained execution region.
method name="VirtualMethod"
declaringType name="VirtualCERCall+MyClass"
  declaringModule name="mda"
    callsite name="MethodWithCer" offset="0x0024"

Configuration

<mdaConfig>
  <assistants>
    <VirtualCERCall />
  </assistants>
</mdaConfig>

Example

class MyClass
{
    [ReliabilityContract(Consistency.MayCorruptProcess, CER.None)]
    virtual void VirtualMethod()
    {
        ...
    }
}

class MyDerivedClass : MyClass
{
    [ReliabilityContract(Consistency.MayCorruptProcess, CER.None)]
    override void VirtualMethod()
    {
        ...
    }
}

void MethodWithCer(MyClass object)
{
    RuntimeHelpers.PrepareConstrainedRegions();
    try
    {
        ...
    }
    finally
    {
        // Start of the CER.

        // Cannot tell at analysis time whether object is a MyClass
        // or a MyDerivedClass, so we do not know which version of
        // VirtualMethod we are going to call.
        object.VirtualMethod();
    }
}

Jaa