更新（EF Core 5.x）：
从版本 5.0 开始，表达式 Select(expr).Distinct().Count() 现在可以被 EF Core 识别并转换为相应的 SQL COUNT(DISTINCT expr))，因此无需修改即可使用原始 LINQ 查询。
原始（EF Core 2.x），由于查询管道重写，该解决方案不适用于 EF Core 3.x：
EF（6 和 Core）历来不支持此标准 SQL 构造。很可能是因为缺乏标准的 LINQ 方法以及将Select(expr).Distinct().Count() 映射到它的技术困难。
好消息是 EF Core 是可扩展的，通过用自定义派生实现替换其许多内部服务来覆盖所需的行为。不容易，需要大量的管道代码，但可行。
所以想法是添加和使用像这样的简单自定义CountDistinct 方法
public static int CountDistinct<T, TKey>(this IQueryable<T> source, Expression<Func<T, TKey>> keySelector)
=> source.Select(keySelector).Distinct().Count();
public static int CountDistinct<T, TKey>(this IEnumerable<T> source, Func<T, TKey> keySelector)
=> source.Select(keySelector).Distinct().Count();
让 EF Core 以某种方式将它们转换为 SQL。事实上，EF Core 提供了一种定义（甚至自定义翻译）数据库标量函数的简单方法，但不幸的是，这不能用于具有单独处理管道的聚合函数。所以我们需要深入挖掘 EF Core 基础架构。
最后提供了 EF Core 2.x 管道的完整代码。不确定是否值得努力，因为 EF Core 3.0 将使用完全重写的查询流程管道。但这很有趣，而且我很确定它可以针对新的（希望更简单的）管道进行更新。
无论如何，您只需将代码复制/粘贴到项目中的新代码文件中，将以下内容添加到上下文OnConfiguring覆盖
optionsBuilder.UseCustomExtensions();
这会将功能插入 EF Core 基础架构，然后像这样查询
var result = db.MyTable
.GroupBy(x => x.PersonID, x => new { VisitStartDate = x.VisitStart.Date })
.Select(g => new
{
Count = g.CountDistinct(x => x.VisitStartDate)
}).ToList();
幸运的是会被翻译成想要的
SELECT COUNT(DISTINCT(CONVERT(date, [x].[VisitStart]))) AS [Count]
FROM [MyTable] AS [x]
GROUP BY [x].[PersonID]
注意聚合方法所需的预选表达式。这是当前所有聚合方法的 EF Core 限制/要求，而不仅仅是我们的。
最后，完成魔术的完整代码：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Internal;
using Microsoft.EntityFrameworkCore.Metadata;
using Microsoft.EntityFrameworkCore.Query;
using Microsoft.EntityFrameworkCore.Query.Expressions;
using Microsoft.EntityFrameworkCore.Query.ExpressionVisitors;
using Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.Internal;
using Microsoft.EntityFrameworkCore.Query.Internal;
using Remotion.Linq;
using Remotion.Linq.Clauses;
using Remotion.Linq.Clauses.ResultOperators;
using Remotion.Linq.Clauses.StreamedData;
using Remotion.Linq.Parsing.Structure.IntermediateModel;
namespace Microsoft.EntityFrameworkCore
{
public static partial class CustomExtensions
{
public static int CountDistinct<T, TKey>(this IQueryable<T> source, Expression<Func<T, TKey>> keySelector)
=> source.Select(keySelector).Distinct().Count();
public static int CountDistinct<T, TKey>(this IEnumerable<T> source, Func<T, TKey> keySelector)
=> source.Select(keySelector).Distinct().Count();
public static DbContextOptionsBuilder UseCustomExtensions(this DbContextOptionsBuilder optionsBuilder)
=> optionsBuilder
.ReplaceService<INodeTypeProviderFactory, CustomNodeTypeProviderFactory>()
.ReplaceService<IRelationalResultOperatorHandler, CustomRelationalResultOperatorHandler>();
}
}
namespace Remotion.Linq.Parsing.Structure.IntermediateModel
{
public sealed class CountDistinctExpressionNode : ResultOperatorExpressionNodeBase
{
public CountDistinctExpressionNode(MethodCallExpressionParseInfo parseInfo, LambdaExpression optionalSelector)
: base(parseInfo, null, optionalSelector) { }
public static IEnumerable<MethodInfo> GetSupportedMethods()
=> typeof(CustomExtensions).GetTypeInfo().GetDeclaredMethods("CountDistinct");
public override Expression Resolve(ParameterExpression inputParameter, Expression expressionToBeResolved, ClauseGenerationContext clauseGenerationContext)
=> throw CreateResolveNotSupportedException();
protected override ResultOperatorBase CreateResultOperator(ClauseGenerationContext clauseGenerationContext)
=> new CountDistinctResultOperator();
}
}
namespace Remotion.Linq.Clauses.ResultOperators
{
public sealed class CountDistinctResultOperator : ValueFromSequenceResultOperatorBase
{
public override ResultOperatorBase Clone(CloneContext cloneContext) => new CountDistinctResultOperator();
public override StreamedValue ExecuteInMemory<T>(StreamedSequence input) => throw new NotSupportedException();
public override IStreamedDataInfo GetOutputDataInfo(IStreamedDataInfo inputInfo) => new StreamedScalarValueInfo(typeof(int));
public override string ToString() => "CountDistinct()";
public override void TransformExpressions(Func<Expression, Expression> transformation) { }
}
}
namespace Microsoft.EntityFrameworkCore.Query.Internal
{
public class CustomNodeTypeProviderFactory : DefaultMethodInfoBasedNodeTypeRegistryFactory
{
public CustomNodeTypeProviderFactory()
=> RegisterMethods(CountDistinctExpressionNode.GetSupportedMethods(), typeof(CountDistinctExpressionNode));
}
public class CustomRelationalResultOperatorHandler : RelationalResultOperatorHandler
{
private static readonly ISet<Type> AggregateResultOperators = (ISet<Type>)
typeof(RequiresMaterializationExpressionVisitor).GetField("_aggregateResultOperators", BindingFlags.NonPublic | BindingFlags.Static)
.GetValue(null);
static CustomRelationalResultOperatorHandler()
=> AggregateResultOperators.Add(typeof(CountDistinctResultOperator));
public CustomRelationalResultOperatorHandler(IModel model, ISqlTranslatingExpressionVisitorFactory sqlTranslatingExpressionVisitorFactory, ISelectExpressionFactory selectExpressionFactory, IResultOperatorHandler resultOperatorHandler)
: base(model, sqlTranslatingExpressionVisitorFactory, selectExpressionFactory, resultOperatorHandler)
{ }
public override Expression HandleResultOperator(EntityQueryModelVisitor entityQueryModelVisitor, ResultOperatorBase resultOperator, QueryModel queryModel)
=> resultOperator is CountDistinctResultOperator ?
HandleCountDistinct(entityQueryModelVisitor, resultOperator, queryModel) :
base.HandleResultOperator(entityQueryModelVisitor, resultOperator, queryModel);
private Expression HandleCountDistinct(EntityQueryModelVisitor entityQueryModelVisitor, ResultOperatorBase resultOperator, QueryModel queryModel)
{
var queryModelVisitor = (RelationalQueryModelVisitor)entityQueryModelVisitor;
var selectExpression = queryModelVisitor.TryGetQuery(queryModel.MainFromClause);
var inputType = queryModel.SelectClause.Selector.Type;
if (CanEvalOnServer(queryModelVisitor)
&& selectExpression != null
&& selectExpression.Projection.Count == 1)
{
PrepareSelectExpressionForAggregate(selectExpression, queryModel);
var expression = selectExpression.Projection[0];
var subExpression = new SqlFunctionExpression(
"DISTINCT", inputType, new[] { expression.UnwrapAliasExpression() });
selectExpression.SetProjectionExpression(new SqlFunctionExpression(
"COUNT", typeof(int), new[] { subExpression }));
return new ResultTransformingExpressionVisitor<int>(
queryModelVisitor.QueryCompilationContext, false)
.Visit(queryModelVisitor.Expression);
}
else
{
queryModelVisitor.RequiresClientResultOperator = true;
var typeArgs = new[] { inputType };
var distinctCall = Expression.Call(
typeof(Enumerable), "Distinct", typeArgs,
queryModelVisitor.Expression);
return Expression.Call(
typeof(Enumerable), "Count", typeArgs,
distinctCall);
}
}
private static bool CanEvalOnServer(RelationalQueryModelVisitor queryModelVisitor) =>
!queryModelVisitor.RequiresClientEval && !queryModelVisitor.RequiresClientSelectMany &&
!queryModelVisitor.RequiresClientJoin && !queryModelVisitor.RequiresClientFilter &&
!queryModelVisitor.RequiresClientOrderBy && !queryModelVisitor.RequiresClientResultOperator &&
!queryModelVisitor.RequiresStreamingGroupResultOperator;
}
}