如何处理 Linq 表达式中的空值

Question

网络核心应用。我实现了通用存储库模式。我正在执行以下某种过滤器功能。

 var param = Expression.Parameter(typeof(SiteAssessmentRequest), "x");
 Expression<Func<SiteAssessmentRequest, bool>> query;
 if(request.role == "Admin")
 {
 query = null;
 }
 else
 {
   query = x => x.CreatedBy == request.Userid || x.AssignedTo == request.Userid;
 }
 Expression body = Expression.Invoke(query, param);
 if (request.Client != null && request.Client.Length != 0)
            {
                Expression<Func<SiteAssessmentRequest, bool>> internalQuery = x => request.Client.Contains(x.Client);
                body = Expression.AndAlso(Expression.Invoke(query, param), Expression.Invoke(internalQuery, param));
            }
  if (request.CountryId != null && request.CountryId.Length != 0)
            {
                Expression<Func<SiteAssessmentRequest, bool>> internalQuery = x => request.CountryId.Contains(x.CountryId);
                body = Expression.AndAlso(Expression.Invoke(query, param), Expression.Invoke(internalQuery, param));
            }
var lambda = Expression.Lambda<Func<SiteAssessmentRequest, bool>>(body, param);
 var siteAssessmentRequest = await _siteAssessmentRequestRepository.GetAsync(lambda, x => x.OrderByDescending(x => x.Status == Status.New.ToString()).ThenByDescending(x => x.Status == GetEnumDescription(Status.InProgress)).ThenByDescending(x => x.Status == Status.Closed.ToString()).ThenByDescending(x => x.StartDate), x => x.Country).ConfigureAwait(false);

在上面的代码中，每当管理员登录时我不想过滤。在 Admin 的情况下，当我将 null 分配给异常后的查询时，它会在下面的行

处抛出

Expression body = Expression.Invoke(query, param);

Parameter: expression is required, it cannot be empty

谁能帮我解决这个问题。任何帮助，将不胜感激。谢谢

Answer 1

我认为你的问题在

if(request.role == "Admin"){ query = null; }
else{
   query = x => x.CreatedBy == request.Userid || x.AssignedTo == request.Userid;
}

本节。当请求来自管理员时，您不想过滤查询。所以，我想你可以试试这个

if(request.role == "Admin")
{ 
   query = x => x.CreatedBy > 0;
}

//If CreatedBy is String Type
if(request.role == "Admin")
{ 
   query = x => !string.IsNullOrEmpty(x.CreatedBy) && !string.IsNullOrWhiteSpace(x.CreatedBy);
}

因为，CreatedBy 应该可用于每个项目。
注意：请尝试一下，如果有效请告诉我。

如何处理 Linq 表达式中的空值

How to handle null in Linq Expression

c#

linq

generics

lambda

repository-pattern