问题
I have the next Code in EF Core 3.1 in language VB.NET
Dim supplierID as string="1545464"
Dim results = (From pa In DC.product.AsNoTracking()
Where pa.supplierID = supplierID
Select pa)
The exception throw is:
The LINQ expression 'DbSet<product>
.Where(p => Operators.CompareString(
Left: p.supplierID,
Right: __$VB$Local_supplierID_0,
TextCompare: False) == 0)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync().
I found the following solution:
Dim supplierID as string="1545464"
Dim results = (From pa In DC.product.AsNoTracking()
Where pa.supplierID.Equals(supplierID)
Select pa)
Is my solution correct, using .Equals()? In C# language if it works with the operator "=="
I have created a small solution with which you can reproduce the error.
The solution has 4 projects:
- Sup.Entities (C#)
- Sup.DAL (C#)
- Sup.ConsoleApp1 (C#)
- Sup.consoleAppVB (VB.NET)
This is the error that occurs in Sup.ConsoleAppVB (VB.NET)
This is the result in Sup.ConsoleApp1 (C#)
Attached solution Download that includes projects and an SQL file to create the database and 1 table with 3 rows.
Please change connectionstring for UseSqlServer("...") in OnConfiguring Context
回答1:
I just ran into this issue but since I have developed my own LINQ to SQL evaluator before I knew how to solve the problem. VB.NET transforms the = operator for strings into a call to Microsoft.VisualBasic.CompilerServices.Operators.CompareString(). So when the expression tree is evaluated this method must be handled. I suspect the reason for this is because VB handles string comparisons to null ((text = Nothing) = True).
I didn't download your sample but I fixed it in an ASP.NET Core application.
If you were using LINQ, this would be handled inside an ExpressionVisitor but for Entity Framework Core 3.1, I found that you can implement an IMethodCallTranslator.
However, Entity Framework uses SqlExpression expressions so an ISqlExpressionFactory is needed to create them. Fortunately, dependency injection can be used to get an implementation from IServiceCollection.
Public Class VbCompareStringMethodCallTranslator : Implements IMethodCallTranslator
Private mExpressionFactory As ISqlExpressionFactory
Public Sub New(expressionFactory As ISqlExpressionFactory)
Me.mExpressionFactory = expressionFactory
End Sub
Public Function Translate(instance As SqlExpression, method As MethodInfo, arguments As IReadOnlyList(Of SqlExpression)) As SqlExpression Implements IMethodCallTranslator.Translate
If method IsNot Nothing Then
If method.Name = "CompareString" AndAlso method.DeclaringType?.Name = "Operators" AndAlso
method.DeclaringType?.Namespace = "Microsoft.VisualBasic.CompilerServices" Then
Dim left = arguments(0)
Dim right = arguments(1)
If method.Name Is NameOf(String.Compare) AndAlso arguments.Count = 2 AndAlso
arguments(0).Type.UnwrapNullableType Is arguments(1).Type.UnwrapNullableType Then
left = arguments(0)
right = arguments(1)
ElseIf method.Name Is NameOf(String.CompareTo) AndAlso arguments.Count = 1 AndAlso
instance IsNot Nothing AndAlso instance.Type.UnwrapNullableType Is arguments(0).Type.UnwrapNullableType Then
left = instance
right = arguments(0)
End If
If left IsNot Nothing AndAlso right IsNot Nothing Then
Return Me.mExpressionFactory.[Case]({New CaseWhenClause(Me.mExpressionFactory.Equal(left, right), Me.mExpressionFactory.Constant(0)),
New CaseWhenClause(Me.mExpressionFactory.GreaterThan(left, right), Me.mExpressionFactory.Constant(1)),
New CaseWhenClause(Me.mExpressionFactory.LessThan(left, right), Me.mExpressionFactory.Constant(-1))},
Nothing)
End If
End If
End If
Return Nothing
End Function
End Class
Making use of the following extension method
Public Module SharedTypeExtensions
<Extension()>
Public Function UnwrapNullableType(type As Type) As Type
Return If(Nullable.GetUnderlyingType(type), type)
End Function
End Module
You can see that this is the code used by Entity Framework to handle string comparisons here https://github.com/dotnet/efcore/blob/3656e9daa9b81398d8c065a702fd5dca91979f49/src/EFCore.Relational/Query/Internal/ComparisonTranslator.cs
So now this needs to be hooked up and the following plumbing code can be used
Public Class VbMethodCallTranslatorPlugin : Implements IMethodCallTranslatorPlugin
Public Sub New(expressionFactory As ISqlExpressionFactory)
Me.Translators = {New VbCompareStringMethodCallTranslator(expressionFactory)}
End Sub
Public ReadOnly Property Translators As IEnumerable(Of IMethodCallTranslator) Implements IMethodCallTranslatorPlugin.Translators
End Class
Public Class VbDbContextOptionsExtension : Implements IDbContextOptionsExtension
Public Sub ApplyServices(services As IServiceCollection) Implements IDbContextOptionsExtension.ApplyServices
services.AddSingleton(Of IMethodCallTranslatorPlugin, VbMethodCallTranslatorPlugin)
End Sub
Public Sub Validate(options As IDbContextOptions) Implements IDbContextOptionsExtension.Validate
End Sub
Public ReadOnly Property Info As DbContextOptionsExtensionInfo Implements IDbContextOptionsExtension.Info
Get
Return New VbDbContextOptionsExtensionInfo(Me)
End Get
End Property
End Class
Public Class VbDbContextOptionsExtensionInfo : Inherits DbContextOptionsExtensionInfo
Public Sub New(extension As IDbContextOptionsExtension)
MyBase.New(extension)
End Sub
Public Overrides Function GetServiceProviderHashCode() As Long
Return Me.Extension.GetHashCode
End Function
Public Overrides Sub PopulateDebugInfo(<NotNullAttribute> debugInfo As IDictionary(Of String, String))
debugInfo("VB:TranslateMethods") = True.ToString
End Sub
Public Overrides ReadOnly Property IsDatabaseProvider As Boolean
Get
Return False
End Get
End Property
Public Overrides ReadOnly Property LogFragment As String
Get
Return "VbMethodSupport=true"
End Get
End Property
End Class
Now this can hooked up using the DbContextOptionsBuilder, but the following extension method will make this easier
Public Module VbDbContextOptionsBuilderExtensions
<Extension>
Public Function AddVbSupport(optionsBuilder As DbContextOptionsBuilder) As DbContextOptionsBuilder
Dim builder = CType(optionsBuilder, IDbContextOptionsBuilderInfrastructure)
Dim extension = If(optionsBuilder.Options.FindExtension(Of VbDbContextOptionsExtension), New VbDbContextOptionsExtension)
builder.AddOrUpdateExtension(extension)
Return optionsBuilder
End Function
End Module
Now you can hook this up while setting up your DbContext
services.AddDbContext(Of ApplicationDbContext)(Sub(options)
options.UseSqlServer(Me.Configuration.GetConnectionString("ConnectionString"),
Sub(dbOptions)
dbOptions.MigrationsAssembly("Database.Migrations")
End Sub)
options.AddVbSupport
End Sub)
Additional Info
This appears to be a bug in Entity Framework rather than VB.NET just not being supported. You can find this code in the dotnet efcore repository. https://github.com/dotnet/efcore/blob/7cb52b388a2d9fd8f9c2c499ef3ffb9753d9932a/src/EFCore/Query/Internal/QueryOptimizingExpressionVisitor.cs#L113-L132
I submitted a bug report here https://github.com/dotnet/efcore/issues/20889
Vote it up so the devs will fix the issue!
Update 1
Looks like this will be fixed in .NET 5
Update 2
The above solution was causing issues after refreshing the page a bunch of times. I would get an error something to the effect of "more than 20 IService instances have been created"
In order to fix this I just added the expression transform into a different part of the pipeline.
Imports System.Linq.Expressions
Imports System.Runtime.CompilerServices
Imports Microsoft.EntityFrameworkCore
Imports Microsoft.EntityFrameworkCore.Query
Public Class VbRelationalQueryTranslationPreprocessorFactory : Implements IQueryTranslationPreprocessorFactory
Private ReadOnly mDependencies As QueryTranslationPreprocessorDependencies
Private ReadOnly mRelationalDependencies As RelationalQueryTranslationPreprocessorDependencies
Public Sub New(dependencies As QueryTranslationPreprocessorDependencies, relationalDependencies As RelationalQueryTranslationPreprocessorDependencies)
Me.mDependencies = dependencies
Me.mRelationalDependencies = relationalDependencies
End Sub
Public Overridable Function Create(queryCompilationContext As QueryCompilationContext) As QueryTranslationPreprocessor Implements IQueryTranslationPreprocessorFactory.Create
Return New VbRelationalQueryTranslationPreprocessor(Me.mDependencies, Me.mRelationalDependencies, queryCompilationContext)
End Function
End Class
Public Class VbRelationalQueryTranslationPreprocessor : Inherits RelationalQueryTranslationPreprocessor
Public Sub New(dependencies As QueryTranslationPreprocessorDependencies, relationalDependencies As RelationalQueryTranslationPreprocessorDependencies, queryCompilationContext As QueryCompilationContext)
MyBase.New(dependencies, relationalDependencies, queryCompilationContext)
End Sub
Public Overrides Function Process(query As Expression) As Expression
query = New LanguageNormalizingExpressionVisitor().Visit(query)
Return MyBase.Process(query)
End Function
End Class
Public Class LanguageNormalizingExpressionVisitor : Inherits ExpressionVisitor
Protected Overrides Function VisitBinary(node As BinaryExpression) As Expression
Dim methodCall = TryCast(node.Left, MethodCallExpression)
If methodCall IsNot Nothing Then
' Replace calls to comparestring with a binary equals on the operands
If methodCall.Method.Name = "CompareString" AndAlso methodCall.Method.DeclaringType?.Name = "Operators" AndAlso methodCall.Method.DeclaringType?.Namespace = "Microsoft.VisualBasic.CompilerServices" Then
Dim left = Me.Visit(methodCall.Arguments(0))
Dim right = Me.Visit(methodCall.Arguments(1))
Return Expression.MakeBinary(node.NodeType, left, right)
End If
End If
Return MyBase.VisitBinary(node)
End Function
End Class
Public Module VbDbContextOptionsBuilderExtensions
<Extension>
Public Function AddVbSupport(optionsBuilder As DbContextOptionsBuilder) As DbContextOptionsBuilder
optionsBuilder.ReplaceService(Of IQueryTranslationPreprocessorFactory, VbRelationalQueryTranslationPreprocessorFactory)()
Return optionsBuilder
End Function
End Module
来源：https://stackoverflow.com/questions/59739568/ef-core-linq-operator-in-vb-net