如何在.NET Core中卸载一个assemlby使其具有收藏价值？

Question

如何在.NET Core中卸载一个assemlby？

注意：.NET Core不支持AppDomains。 .NET Core不支持AppDomains。 背景：我需要评估用户生成的程序集。 我必须动态地评估用户生成的VisualBasic表达式。 为此，我使用Roslyn动态编译表达式。 我从Roslyn编译器生成的字节数组中加载生成的汇编。 然后我创建了一个实现抽象类的实例（所以我不必使用反射）。然后我调用抽象类的EvaluateExpression方法。 在这之后，我想卸载加载的程序集（否则，我将会有内存泄漏的乐趣）。 所以，我在评估完表达式之后，马上卸载了这个程序集。

Parameters.AbstractEvaluator x = RoslynExpressionEvaluator.CreateEvaluator(expression, report.Code);
object value = x.EvaluateExpression();
x.LoadContext.Unload();

(loadContext在生成时被保存在抽象类中)

到目前为止，一切都很好，但在 x.LoadContext.Unload();，我得到

System.InvalidOperationException. "无法卸载不可收集的AssemblyLoadContext. "无法卸载不可收集的AssemblyLoadContext."

能否解决这个问题？ 我怎样才能使一个汇编成为可收集的？ 另外，我注意到我可以用相同的类名加载一个汇编（如你所见，代码中没有命名空间），这在多线程环境（也就是web）中的表现如何？ 我是否可以无休止地加载和加载不同版本的动态生成类，直到机器耗尽内存而不发生故障？或者说，为什么当加载同一个类两次时，这个方法还能用？

using Microsoft.CodeAnalysis.Operations;

namespace ReportTester
{


    public static class RoslynExpressionEvaluator
    {
        // a utility method that creates Roslyn compilation
        // for the passed code. 
        // The compilation references the collection of 
        // passed "references" arguments plus
        // the mscore library (which is required for the basic
        // functionality).
        private static Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilation
            CreateCompilationWithMscorlib
            (
                string assemblyOrModuleName,
                string code,
                Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilationOptions compilerOptions = null,
                System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.MetadataReference> references = null)
        {
            // create the syntax tree
            Microsoft.CodeAnalysis.SyntaxTree syntaxTree =
                Microsoft.CodeAnalysis.VisualBasic.SyntaxFactory.ParseSyntaxTree(code, null, "");

            // get the reference to mscore library
            Microsoft.CodeAnalysis.MetadataReference mscoreLibReference =
                Microsoft.CodeAnalysis.AssemblyMetadata
                    .CreateFromFile(typeof(string).Assembly.Location)
                    .GetReference();

            // create the allReferences collection consisting of 
            // mscore reference and all the references passed to the method
            System.Collections.Generic.List<Microsoft.CodeAnalysis.MetadataReference> allReferences =
                new System.Collections.Generic.List<Microsoft.CodeAnalysis.MetadataReference>() { mscoreLibReference  };

            if (references != null)
            {
                allReferences.AddRange(references);
            } // End if (references != null) 

            // create and return the compilation
            Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilation compilation =
                Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilation.Create
                (
                    assemblyOrModuleName,
                    new[] {syntaxTree},
                    options: compilerOptions,
                    references: allReferences
                );

            return compilation;
        } // End Function CreateCompilationWithMscorlib 


        // emit the compilation result into a byte array.
        // throw an exception with corresponding message
        // if there are errors
        private static byte[] EmitToArray( this Microsoft.CodeAnalysis.Compilation compilation )
        {
            using (System.IO.MemoryStream stream = new System.IO.MemoryStream())
            {
                // emit result into a stream
                Microsoft.CodeAnalysis.Emit.EmitResult emitResult = compilation.Emit(stream);

                if (!emitResult.Success)
                {
                    // if not successful, throw an exception
                    foreach (Microsoft.CodeAnalysis.Diagnostic thisError in emitResult.Diagnostics)
                    { 
                        if(thisError.Severity == Microsoft.CodeAnalysis.DiagnosticSeverity.Error)
                            throw new System.Exception(thisError.GetMessage());
                    } // Next thisError 

                } // End if (!emitResult.Success) 

                // get the byte array from a stream
                return stream.ToArray();
            } // End Using stream 

        } // End Function EmitToArray 


        public static Parameters.AbstractEvaluator CreateEvaluator(string expression, string code)
        {
            try
            {
                // the main class Program contain static void Main() 
                // that calls A.Print() and B.Print() methods
                string mainProgramString = @"
Option Strict Off
Option Explicit Off
Option Infer On

Imports ReportTester.Parameters

imports System
Imports System.Collections.Generic
Imports Microsoft.VisualBasic.Strings
Imports Microsoft.VisualBasic.Interaction
Imports Microsoft.VisualBasic.Information



Public Class CodeImplementation
" + code + @"
End Class ' CodeImplementation




Public Class RsEval
    Inherits AbstractEvaluator


    Public Code As CodeImplementation


    Public Sub New()
        Me.New(New ParameterCollection)
    End Sub


    Public Sub New(ByVal allParameters As ParameterCollection)
        MyBase.New(allParameters)
        'code
    End Sub


    Public Overrides Function EvaluateExpression() As Object
        Return " + expression + @"
    End Function


End Class ' RsEval

";

                Microsoft.CodeAnalysis.MetadataReference sysRuntime =
                    Microsoft.CodeAnalysis.MetadataReference.CreateFromFile(
                        typeof(System.Runtime.AssemblyTargetedPatchBandAttribute).Assembly.Location);

                Microsoft.CodeAnalysis.MetadataReference vbRuntime =
                    Microsoft.CodeAnalysis.MetadataReference.CreateFromFile(typeof(Microsoft.VisualBasic.Constants)
                        .Assembly
                        .Location);

                Microsoft.CodeAnalysis.MetadataReference sysCorlib =
                    Microsoft.CodeAnalysis.MetadataReference.CreateFromFile(typeof(object).Assembly.Location);

                Microsoft.CodeAnalysis.MetadataReference sysConsole =
                    Microsoft.CodeAnalysis.MetadataReference.CreateFromFile(typeof(System.Console).Assembly.Location);


                Microsoft.CodeAnalysis.MetadataReference reportParameters =
                    Microsoft.CodeAnalysis.MetadataReference.CreateFromFile(typeof(ReportTester.Parameters.ParameterValue).Assembly.Location);


                Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilationOptions co = 
                    new Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilationOptions
                    (
                        Microsoft.CodeAnalysis.OutputKind.DynamicallyLinkedLibrary
                    );


                co.WithOptionStrict(Microsoft.CodeAnalysis.VisualBasic.OptionStrict.Off);
                co.WithOptionExplicit(false);
                co.WithOptionInfer(true);


                // create the Roslyn compilation for the main program with
                // ConsoleApplication compilation options
                // adding references to A.netmodule and B.netmodule
                Microsoft.CodeAnalysis.VisualBasic.VisualBasicCompilation mainCompilation =
                    CreateCompilationWithMscorlib
                    (
                        "program",
                        mainProgramString,
                        // note that here we pass the OutputKind set to ConsoleApplication
                        compilerOptions: co,
                        references: new[] {sysRuntime, vbRuntime, sysCorlib, sysConsole, reportParameters }
                    );

                // Emit the byte result of the compilation
                byte[] result = mainCompilation.EmitToArray();

                // System.AppDomain temporaryAppDomain = System.AppDomain.CreateDomain("TemporaryAppDomain");
                // System.Reflection.Assembly assembly = temporaryAppDomain.Load(result);
                // not supported ... 

                // Load the resulting assembly into the domain. 
                System.Reflection.Assembly assembly = System.Reflection.Assembly.Load(result);


                // here we get the Program type and 
                // call its static method Main()
                // to test the program. 

                // get the type Program from the assembly
                System.Type programType = assembly.GetType("RsEval");




                Parameters.AbstractEvaluator x = (Parameters.AbstractEvaluator)System.Activator.CreateInstance(programType);
                x.LoadContext = System.Runtime.Loader.AssemblyLoadContext.GetLoadContext(assembly);


                // Get the static Main() method info from the type
                // System.Reflection.MethodInfo method = programType.GetMethod("EvaluateExpression");
                // invoke Program.Main() static method
                // object retValue = method.Invoke(null, null);

                // System.AppDomain.Unload(temporaryAppDomain);

                return x;
            }
            catch (System.Exception ex)
            {
                System.Console.WriteLine(ex.ToString());
                throw;
            }

            return null;
        } // End Sub Test 


    } // End Class RoslynExpressionEvaluator 


} // End Namespace ReportTester 

抽象类。

Public MustInherit Class AbstractEvaluator

    Public Parameters As ParameterCollection

    Public LoadContext As System.Runtime.Loader.AssemblyLoadContext


    Public Sub New()
        Me.New(New ParameterCollection)
    End Sub


    Public Sub New(ByVal allParameters As ParameterCollection)
        Me.Parameters = allParameters
    End Sub

    Public Overridable Sub SetValue(ByVal parameterName As String, parameter As ReportParameter)
        Me.Parameters.Parameters(parameterName) = parameter
    End Sub

    Public Overridable Function GetValue(ByVal parameterName As String) As ReportParameter
        Return Me.Parameters.Parameters(parameterName)
    End Function


    Public Overridable Sub SetParameters(ByVal allParameters As ParameterCollection)
        Me.Parameters = Nothing
        Me.Parameters = allParameters
    End Sub


    Public MustOverride Function EvaluateExpression() As Object



    '  Public Parameters As ParameterCollection
    ' Public MustOverride Sub SetCompulsoryParameter()


End Class ' AbstractEvaluator


' example
Public Class ConcreteEvaluator
    Inherits AbstractEvaluator



    Public Sub New()
        Me.New(New ParameterCollection)
    End Sub


    Public Sub New(ByVal allParameters As ParameterCollection)
        MyBase.New(allParameters)
        'code
    End Sub



    'Public Overrides Sub SetCompulsoryParameter()
    'End Sub

    Public Overrides Function EvaluateExpression() As Object
        Dim expression As String = "System.DateTime.Now.AddDays(1+2+3).ToString(""dd.MM.yyyy"")" ' string expression would come from report, compiled with roslyn
        Return " + expression + @"
    End Function


End Class

Answer 1

嗯，问题是它会卸载整个上下文。 这意味着，如果我把程序集加载到默认上下文中，又称System.Runtime.Loader.AssemblyLoadContext.Default.LoadFromStream(ms);然后，在卸载时，它会卸载整个默认上下文。 绝对不是好事... ==>

System.InvalidOperationException: "无法卸载不可收集的AssemblyLoadContext."

所以你必须在不同的上下文（类似于AppDomain）中加载装配体

public class CollectibleAssemblyLoadContext : AssemblyLoadContext
{
    public CollectibleAssemblyLoadContext() : base(isCollectible: true)
    { }

    protected override Assembly Load(AssemblyName assemblyName)
    {
        return null;
    }
}

byte[] result = null; // Assembly Emit-result from roslyn
System.Runtime.Loader.AssemblyLoadContext context = new CollectibleAssemblyLoadContext();
System.IO.Stream ms = new System.IO.MemoryStream(result);
System.Reflection.Assembly assembly = context.LoadFromStream(ms);


System.Type programType = assembly.GetType("RsEval");
MyAbstractClass eval = (MyAbstractClass) System.Activator.CreateInstance(programType);
eval.LoadContext = context;
eval.Stream = ms;
// do something here with the dynamically created class "eval"

然后你可以说

eval.LoadContext.Unload();
eval.Stream.Dispose();

如果你把它放到抽象类的IDisposable接口中，那么你就可以直接使用，如果你想的话。

using (Parameters.AbstractEvaluator x = RoslynExpressionEvaluator.CreateEvaluator(expression, report.Code))
{
    object value = x.EvaluateExpression();
    System.Console.WriteLine(value);
}

所以这个抽象类看起来是这样的。

Public MustInherit Class AbstractEvaluator
    Implements IDisposable

    Public Parameters As ParameterCollection

    Public LoadContext As System.Runtime.Loader.AssemblyLoadContext
    Public Stream As System.IO.Stream



    Private disposedValue As Boolean ' Dient zur Erkennung redundanter Aufrufe.

    Protected Overridable Sub Dispose(disposing As Boolean)
        If Not disposedValue Then
            If disposing Then
                ' TODO: verwalteten Zustand (verwaltete Objekte) entsorgen.

                If Me.LoadContext IsNot Nothing Then
                    Me.LoadContext.Unload()
                    Me.LoadContext = Nothing
                End If

                If Me.Stream IsNot Nothing Then
                    Me.Stream.Dispose()
                    Me.Stream = Nothing
                End If

            End If

            ' TODO: nicht verwaltete Ressourcen (nicht verwaltete Objekte) freigeben und Finalize() weiter unten überschreiben.
            ' TODO: große Felder auf Null setzen.
        End If
        disposedValue = True
    End Sub

    ' TODO: Finalize() nur überschreiben, wenn Dispose(disposing As Boolean) weiter oben Code zur Bereinigung nicht verwalteter Ressourcen enthält.
    'Protected Overrides Sub Finalize()
    '    ' Ändern Sie diesen Code nicht. Fügen Sie Bereinigungscode in Dispose(disposing As Boolean) weiter oben ein.
    '    Dispose(False)
    '    MyBase.Finalize()
    'End Sub

    ' Dieser Code wird von Visual Basic hinzugefügt, um das Dispose-Muster richtig zu implementieren.
    Public Sub Dispose() Implements IDisposable.Dispose
        ' Ändern Sie diesen Code nicht. Fügen Sie Bereinigungscode in Dispose(disposing As Boolean) weiter oben ein.
        Dispose(True)
        ' TODO: Auskommentierung der folgenden Zeile aufheben, wenn Finalize() oben überschrieben wird.
        ' GC.SuppressFinalize(Me)
    End Sub



    Public Sub New()
        Me.New(New ParameterCollection)
    End Sub


    Public Sub New(ByVal allParameters As ParameterCollection)
        Me.Parameters = allParameters
    End Sub

    Public Overridable Sub SetValue(ByVal parameterName As String, parameter As ReportParameter)
        Me.Parameters.Parameters(parameterName) = parameter
    End Sub

    Public Overridable Function GetValue(ByVal parameterName As String) As ReportParameter
        Return Me.Parameters.Parameters(parameterName)
    End Function


    Public Overridable Sub SetParameters(ByVal allParameters As ParameterCollection)
        Me.Parameters = Nothing
        Me.Parameters = allParameters
    End Sub


    Public MustOverride Function EvaluateExpression() As Object

    ' Public Parameters As ParameterCollection
    ' Public MustOverride Sub SetCompulsoryParameter()


End Class ' AbstractEvaluator


' example
Public Class ConcreteEvaluator
    Inherits AbstractEvaluator

    Class SimplisticExampleCode

        Public Function Tomorrow() As System.DateTime
            Return System.DateTime.Now.AddDays(1)
        End Function

    End Class

    Friend Code As SimplisticExampleCode


    Public Sub New()
        Me.New(New ParameterCollection)
    End Sub


    Public Sub New(ByVal allParameters As ParameterCollection)
        MyBase.New(allParameters)
        'code
        Me.Code = New SimplisticExampleCode
    End Sub



    'Public Overrides Sub SetCompulsoryParameter()
    'End Sub

    Public Overrides Function EvaluateExpression() As Object
        'Dim expression As String = "System.DateTime.Now.AddDays(1+2+3).ToString(""dd.MM.yyyy"")" ' string expression would come from report, compiled with roslyn
        'Return " + expression + @"
        Return Code.Tomorrow().ToString("dd.MM.yyyy")
    End Function


End Class