EF Core中Expression Visitor改写查询时访问局部变量参数属性报错问题

场景说明 #

我尝试用Expression Visitor改写EF Core查询，将初始的对象相等判断，转换成逐个属性匹配的形式。

初始查询代码：

var contractSubjectKey = new ContractSubjectKey(subjectId, rank);

insurance = context.Set<Insurance>()
    .FirstOrDefault(i => i.Key.ContractSubjectKey == contractSubjectKey);

预期改写后的表达式：

i => i.Key.ContractSubjectKey.Id == contractSubjectKey.Id

遇到的问题 #

左侧的MemberExpression属性访问处理正常，但右侧如果是代表局部变量的ParameterExpression，使用var right = Expression.Property(parameterExpression, "Id");生成表达式后，会触发LINQ表达式无法翻译的错误：

System.InvalidOperationException : The LINQ expression 'DbSet()
.Where(i => EF.Property(EF.Property(i, "Key"), "ContractSubjectKey").Id == __contractSubjectKey_0.Id)' could not be translated.

问题排查与解决思路 #

后续发现问题根源在于EF Core的IQueryTranslationPreprocessor已经提前提取并缓存了参数，等到IQueryExpressionInterceptor阶段再进行这类改写就无法被正确处理了。目前计划通过自定义IQueryTranslationPreprocessor或Postprocessor来解决该问题，但需要关注性能开销。

相关上下文代码 #

public record ContractSubjectKey(int Id, int Rank);

//insurance.Key.ContractSubjectKey 实际是 ContractSubjectKeyInsurance，继承自 ContractSubjectKey
public record ContractSubjectKeyInsurance : ContractSubjectKey
{
    public ContractSubjectKeyInsurance(int Id, int Rank) : base(Id, Rank)
    {

    }
    public ContractSubjectKeyInsurance(int Id, int Rank, int InsuranceKeyId) : base(Id, Rank)
    {
        _insuranceKeyId = InsuranceKeyId;
    }

    private int _insuranceKeyId;
}

public record InsuranceKey(int Id, ContractSubjectKey ContractSubjectKey)    
{
    this.Id = Id;
    _contractSubjectId = ContractSubjectKey.Id;
    _contractSubjectRank = ContractSubjectKey.Rank;
    this.ContractSubjectKey = 
       new ContractSubjectKeyInsurance(ContractSubjectKey.Id,  ContractSubjectKey.Rank, Id);
}

内容的提问来源于stack exchange，提问作者Matus