c# - 无法从表达式中提取此函数

Question

我正在尝试参数化 Entity Framework 查询 (EntityFrameworkCore 1.1.1)，但在从“Where”表达式中提取函数时出现错误:

System.NotSupportedException :

Could not parse expression 'school.Programs.Any(__p_0)': The given arguments did not match the expected arguments: Object of type 'System.Linq.Expressions.TypedParameterExpression' cannot be converted to type 'System.Linq.Expressions.LambdaExpression'.

当我尝试调用 SearchSchools() 时出现该错误:

public IQueryable<SchoolResult> SearchSchoolQueryable(
    Expression<Func<School, bool>> schoolExpression)
{
    return _dbContext.School
        .Where(schoolExpression)
        .Select(s => new SchoolResult
            {
                Id = s.Id,
                Name = s.Name
            }
         );
}

// hardcoded test: doesn't work
public async Task<IEnumerable<SchoolResult>> SearchSchools()
{
    Func<Program, bool> p = program => program.Name.StartsWith("U");
    Expression<Func<School, bool>> expr = school => school.Active 
        && school.Programs.Any(p);
    return await SearchSchoolQueryable(expr)
        .ToListAsync()
}

但是，当我重写 SearchSchools 以便内联 Func 时它工作正常:

// hardcoded test: works ok
public async Task<IEnumerable<SchoolResult>> SearchSchools()
{
    Expression<Func<School, bool>> expr = school => school.Active 
         && school.Programs.Any(program => program.Name.StartsWith("U"));
    return await SearchSchoolQueryable(expr)
          .ToListAsync()
}

这似乎有点奇怪——我认为他们应该采取同样的行动。为什么会发生这种情况？

Answer 1

第一种情况的错误是你声明了p作为 Func<Program, bool> .

它必须是 Expression<Func<Program, bool>> Entity Framework 理解表达式树。