如何声明带有复杂查询的System.Linq.IQueryable类型变量？

The error you're seeing happens because IQueryable<T> is a generic interface that only accepts one type argument—you can't pass multiple types directly like you tried. To solve this, you have two solid options depending on your needs:

Option 1: Create a Custom Class (Recommended for Reusability) #

If this result type will be used in multiple parts of your codebase, defining a concrete class makes your code more readable and maintainable.

First, define a class that matches the structure of your query's output:

public class SchemeExpenditureResult
{
    public string ProjectName { get; set; }
    public int ADPNo { get; set; }
    public decimal Allocation { get; set; }
    public IEnumerable<IGrouping<string, Expenditure>> Expenditures { get; set; }
}

Then declare your result variable using this class as the type argument:

System.Linq.IQueryable<SchemeExpenditureResult> result = null;

Finally, update your LINQ query to instantiate this class instead of using an anonymous type:

result = from sch in schemeDashboard 
         join exp in Expenditure on sch.schemeId equals exp.SchemeCode into SchExpGroup 
         where sch.SectorDepartmentId == selectedDepartmentId && sch.YearCode == StateManager.CurrentYear 
         orderby sch.ADPId 
         select new SchemeExpenditureResult 
         { 
             ProjectName = sch.schemeName, 
             ADPNo = sch.ADPId, 
             Allocation = sch.CurrentAllocation, 
             Expenditures = from expend in SchExpGroup 
                            where expend.YearCode == StateManager.CurrentYear && expend.DepartmentId == selectedDepartmentId && InvStatus.Contains(expend.Status) 
                            orderby expend.ADPId 
                            group expend by expend.InvoiceId 
         };

Option 2: Use a Value Tuple (Quick for One-Off Use) #

If this is a one-off scenario and you're using C# 7.0 or later, you can use a value tuple to avoid creating a separate class.

Declare your result variable with a named tuple type:

System.Linq.IQueryable<(string ProjectName, int ADPNo, decimal Allocation, IEnumerable<IGrouping<string, Expenditure>> Expenditures)> result = null;

Then adjust your LINQ query to return the tuple:

result = from sch in schemeDashboard 
         join exp in Expenditure on sch.schemeId equals exp.SchemeCode into SchExpGroup 
         where sch.SectorDepartmentId == selectedDepartmentId && sch.YearCode == StateManager.CurrentYear 
         orderby sch.ADPId 
         select (
             ProjectName: sch.schemeName, 
             ADPNo: sch.ADPId, 
             Allocation: sch.CurrentAllocation, 
             Expenditures: from expend in SchExpGroup 
                            where expend.YearCode == StateManager.CurrentYear && expend.DepartmentId == selectedDepartmentId && InvStatus.Contains(expend.Status) 
                            orderby expend.ADPId 
                            group expend by expend.InvoiceId 
         );

Which Option to Choose? #

• Custom Class: Best if you need to reuse this type elsewhere, or if you want to add methods/validation to the result.
• Value Tuple: Faster to implement for single-use cases, keeps your code concise without extra class definitions.

内容的提问来源于stack exchange，提问作者AKA