EF核心:无法为具有泛型StartsWith()表达式的嵌套列表转换LINQ表达式

Question

我正在尝试构建一种通用的"StartsWith"；调用">OtherListEntities"的嵌套列表中的表达式。被管理实体如下所示：

public class MyEntity
{
[System.ComponentModel.DataAnnotations.Key] // key just for the sake of having a key defined, not relevant for the question
public string TopLevelString { get; set; }
public IList<AnotherEntity> OtherListEntities { get; set; }
}
public class AnotherEntity
{
[System.ComponentModel.DataAnnotations.Key]  // key just for the sake of having a key defined, not relevant for the question
public string NestedString { get; set; }
}

我把它放在这样一个上下文中：

public class MyDbContext : DbContext
{
public DbSet<MyEntity> MyEntities { get; set; }
public MyDbContext(DbContextOptions<MyDbContext> options) {}
protected override void OnConfiguring(DbContextOptionsBuilder options) => options.UseSqlite("Data Source=sqlitedemo.db");
}

并尝试在测试类中使用此上下文：

public partial class MyTestClass
{
private List<MyEntity> testExampleList = new List<MyEntity>()
{
new MyEntity()
{
TopLevelString = "ABC",
OtherListEntities = new List<AnotherEntity>()
{
new AnotherEntity(){ NestedString = "ASD"},
new AnotherEntity(){ NestedString = "DEF"},
new AnotherEntity(){ NestedString = "GHI"},
}
},
new MyEntity()
{
TopLevelString = "XYZ",
OtherListEntities = new List<AnotherEntity>()
{
new AnotherEntity(){ NestedString = "asd"},
new AnotherEntity(){ NestedString = "XXX"},
new AnotherEntity(){ NestedString = "FHJ"},
}
}
};
MyDbContext context;
public MyTestClass()
{
var options = new DbContextOptions<MyDbContext>();
this.context = new MyDbContext(options);
context.Database.EnsureDeleted();
context.Database.EnsureCreated();
this.context.MyEntities.AddRange(testExampleList);
this.context.SaveChanges();
}
}

这是我想做的一件简单的事情Where过滤器：

public partial class MyTestClass
{
[Fact]
public void TestFilteringWithoutOwnExpression()
{
Assert.Equal(1, context.MyEntities.Where(x => x.TopLevelString.StartsWith("A")).Count()); // works fine
Assert.Equal(1, context.MyEntities.Where(x => x.OtherListEntities.Where(e => e.NestedString.StartsWith("XXX")).Any()).Count());
}
}

由于在实际应用where子句之前还有其他一些神奇的事情发生，我试图将其包装成一个自己的表达式，如下所示：

public partial class MyTestClass
{
[Fact]
public void TestFilteringWithExpression()
{
Assert.Equal(1, context.MyEntities.MyWhere<MyEntity>(x => x.TopLevelString, "A").Count()); // works
}
[Fact]
public void TestFilteringWithExpressionAny()
{
Assert.Equal(1, context.MyEntities.Where(x => x.OtherListEntities.AsQueryable().MyAny(e => e.NestedString, "XXX")).Count()); // doesn't work, why?
}
}

MyWhere((和MyAny((在扩展类中定义

public static class IQueryableExtension
{
public static IQueryable<TEntity> MyWhere<TEntity>(this IQueryable<TEntity> query, Expression<Func<TEntity, string>> stringSelector, string searchString)
{
ParameterExpression entityParameter = stringSelector.Parameters.First(); //Expression.Parameter(typeof(TEntity), stringSelector.Parameters.First().Name);
MemberExpression memberExpr = (MemberExpression)(stringSelector.Body);
var searchConstant = Expression.Constant(searchString, typeof(string));
var filterExpression = Expression.Lambda<Func<TEntity, bool>>(
Expression.Call(
memberExpr,
typeof(string).GetMethod(nameof(string.StartsWith), new Type[] { typeof(string) }),
searchConstant),
entityParameter);
query = query.Where(filterExpression);
return query;
}
public static bool MyAny<TEntity>(this IQueryable<TEntity> query, Expression<Func<TEntity, string>> stringSelector, string searchString)
{
return query.MyWhere(stringSelector, searchString).Any();
}
}

代码失败，出现InvalidOperationException：

LINQ表达式的DbSet.其中(m=>DbSet.其中(a=>EF.Property(m，"TopLevelString"(！=空&amp；EF.Property(m，"TopLevelString"(==EF.Property(a，"MyEntityTopLevelString"(.MyAny(查询：e=>e.NestedString，字符串选择器："；XXX"(’无法翻译。以可翻译的形式重写查询，或者切换通过插入对AsEnumerable((、AsAsyncEnumerable((、ToList((或ToListAsync((。看见https://go.microsoft.com/fwlink/?linkid=2101038了解更多信息。

如何在嵌套列表中设置通用且可翻译的StartsWith表达式？

Answer 1

如果您只需要对具有以XXX开头的AnotherEntity的MyEntities进行计数，您可以简单地写：

Assert.Equal(1, context.MyEntities.SelectMany(x => x.OtherListEntities).Where(o => o.NestedString.StartsWith("XXX")).Count();

要理解为什么您的扩展方法不起作用，请参阅以下答案。