如何将给定的 lambda 查询转换为 IQueryable

Question

我准备了这个 Linq-to-SQL 查询，它需要以 IQueryable 执行，但它失败了。

当我通过调用ToList 转换filteredResult 时，它运行良好，但我需要使用filteredResult 作为IQueryable 并获得如下选择结果。但我得到一个错误

带有语句体的 lambda 表达式不能转换为表达式树

通过下面的链接，但是，如何将下面的查询转换为IQueryable。根据链接，我是否需要写 Func<object,object> 或任何转换查询的示例真的很有帮助。

"A lambda expression with a statement body cannot be converted to an expression tree"

var result = filteredResult.Select(g => {
    var type1 = g.FirstOrDefault(x => x.CategoryId == (int)CategoryEnum.Typ1);
    var type2 = g.FirstOrDefault(x => x.CategoryId == (int)CategoryEnum.Typ2);

return new AutoDetailDto
    {
        MilestoneId = g.Key.MilestoneId,
        MilestoneName = g.Key.MilestoneName,
        PGrade = type1?.GDR,
        PGradeChange = type1?.HighestGDR
        QGrade = type2.GDR,
        QGradeChange = type2?.HighestGDR    
    };
});

Answer 1

单独的功能将无济于事。无论哪种情况，系统都无法确定如何从复杂的 C# 代码生成 SQL。

试试这个：

// This is converted to SQl because the lambda, though compe, is a single statement
var dataResult = filteredResult.Select(g => new {
    type1 = g.FirstOrDefault(x => x.CategoryId == (int)CategoryEnum.Typ1),
    type2 = g.FirstOrDefault(x => x.CategoryId == (int)CategoryEnum.Typ2),
    MilestoneId  = g.Key.MilestoneId,
    MilestoneName = g.Key.MilestoneName
}).ToArray(); // ToArray() forces execution of the query  

// This select is done entirely in memory
var result = dataResult.Select (e =>
       new AutoDetailDto
       {
           MilestoneId = e.MilestoneId,
           MilestoneName = e.MilestoneName,
           PGrade = e.type1?.GDR,
           PGradeChange = e.type1?.HighestGDR
           QGrade = e.type2.GDR,
           QGradeChange = e.type2?.HighestGDR    
       });

我认为这应该可行。告诉我。

编辑：

奇怪的是，虽然 null 传播运算符是不允许的，但旧的条件 -if 是，所以尝试：

var dataResult = filteredResult.Select(g => new {
        type1 = g.FirstOrDefault(x => x.CategoryId == (int)CategoryEnum.Typ1),
        type2 = g.FirstOrDefault(x => x.CategoryId == (int)CategoryEnum.Typ2),
        MilestoneId  = g.Key.MilestoneId,
        MilestoneName = g.Key.MilestoneName
    }); // No forcing of execution here

var result = dataResult.Select (e =>
       new AutoDetailDto
       {
           MilestoneId = e.MilestoneId,
           MilestoneName = e.MilestoneName,
           PGrade = e.type1 == null ? null : e.type1.GDR,
           PGradeChange = e.type1 == null ? null :e.type1.HighestGDR
           QGrade = e.type2 == null ? null : e.type2.GDR,
           QGradeChange = e.type2 == null ? null : e.type2.HighestGDR    
       });