EF Core：通用 StartsWith() 表达式不可翻译

Question

我正在尝试在 EF Core 3.1.5 中构建一种通用的“StartsWith”表达式。托管实体如下所示：

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 AnotherEntity OtherEntity { 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; }
}

我把它放在这样的上下文中：

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");
}

并尝试在测试中使用此上下文 class:

public partial class MyTestClass // this part is just to make the example 100% reproducable
{ 
    // define some minimal examples to work with
    private List<MyEntity> testExampleList = new List<MyEntity>()
    {
        new MyEntity()
        {
            TopLevelString = "ABC",
            OtherEntity = new AnotherEntity(){NestedString = "ABC"}
        },
        new MyEntity()
        {
            TopLevelString = "XYZ",
            OtherEntity = new AnotherEntity(){NestedString = "XYZ"}
        }
    };

    MyDbContext context;
    public MyTestClass()
    {
        // set up database
        var options = new DbContextOptions<MyDbContext>();
        this.context = new MyDbContext(options);
        context.Database.EnsureDeleted();
        context.Database.EnsureCreated();

        // add examples from above list
        this.context.MyEntities.AddRange(testExampleList);
        this.context.SaveChanges();
    }
}

这是我想作为普通 Where 过滤器做的事情：

public partial class MyTestClass // this part works as expected and is just used to illustrate the purpose of below code
{ 
    [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.OtherEntity.NestedString.StartsWith("A")).Count()); // works, too
    }
}

因为在应用实际的 where 子句之前应该会发生一些其他的魔法，我试图将它包装成一个自己的表达式，如下所示：

public partial class MyTestClass // this part does not work and I don' know why
{
    [Fact]
    public void TestFilteringWithExpression()
    {
        Assert.Equal(1, context.MyEntities.MyWhere<MyEntity>(x => x.TopLevelString, "A").Count());
        Assert.Equal(1, context.MyEntities.MyWhere<MyEntity>(x => x.OtherEntity.NestedString, "A").Count());
    }
}

with MyWhere 在扩展中定义 class:

public static class IQueryableExtension
{
    public static IQueryable<TEntity> MyWhere<TEntity>(this IQueryable<TEntity> query, Expression<Func<TEntity, string>> stringSelector, string searchString)
    {
        ParameterExpression entityParameter = 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;
    }
}

我看到了类似的示例，其中使用了 PropertyExpression 而不是 MemberExpression，但是当我不仅尝试访问 MyEntity.TopLevelString 还尝试访问嵌套的 MyEntity.AnotherEntity.NestedString.

代码失败并出现 InvalidOperationException：

The LINQ expression 'DbSet .Where(m => x.TopLevelString != null && "A" != null && x.TopLevelString.StartsWith("A"))' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation...+

如何设置通用且可翻译的 StartsWith 表达式？

Answer 1

嗯，这个问题与StartsWith无关。如果您检查异常 DbSet .Where(m => x.TopLevelString != null && "A" != null && x.TopLevelString.StartsWith("A") 以 m => 开头但内部使用 x 这是由 ParameterExpression entityParameter = Expression.Parameter(typeof(TEntity), stringSelector.Parameters.First().Name); 行引起的。您使用该参数名称生成另一个参数，但它们不相同。

有 2 个解决方案。

就是直接使用参数，在上面扩展你的表达式
用新创建的多余参数重写您的成员表达式。

所以基本上这将解决您的问题:

ParameterExpression entityParameter =  stringSelector.Parameters.First();

如果你在像 List<T> 这样的集合上尝试你的扩展方法，你会得到如下错误：

System.InvalidOperationException: 'variable 'x' of type 'ExpressionTest.MyEntity' referenced from scope '', but it is not defined'

EF Core：通用 StartsWith() 表达式不可翻译

EF Core: Generic StartsWith() Expression not Translatable

c#

linq-to-sql

ef-core-3.1