IMHO你应该是这样的：

  Database DB = new Database（）; 
 var result = DB.SomeClass.Where（x => 
 Number == x.Class1.SomeNumber || 
 Number == x.Class2.SomeNumber || 
 Number == x.Class3.SomeNumber）
 .ToList（）; 
 

您的查询加载所有数据，之后您将评估.NET中的条件=您必须先检查空值访问 SomeNumber ，但如果您通过Linq到实体评估SQL中的 SomeNumber ，则不需要。 Linq-to-entities应该执行自动的空合并。

I use EntityFramework with POCOs.
Suppose I have POCOs defined like this (simplified):

class Class1
{
    public int ID;
    public int SomeNumber;
}

class Class2
{
    public int ID;
    public int SomeNumber;
}

class Class3
{
    public int ID;
    public int SomeNumber;
}

class SomeClass
{
    public int ID;
    public int? Class1ID;
    public Class1 Class1;
    public int? Class2ID;
    public Class2 Class2;
    public int? Class3ID;
    public Class3 Class3;
}

I want to fetch all SomeClass records from the database, that belong to an either one of Class1, Class2 or Class3 where ClassX.SomeNumber equals some number.

I wrote the LINQ query which looks like this:

Database DB = new Database(); // object context
var result = DB.SomeClass.ToList();

int SomeNumber = 1; // some number
List<SomeClass> retValue = result
    .Where(x =>
        {
            int Number = 0;
            if (x.Class1 != null)
                Number = x.Class1.SomeNumber;
            else if (x.Class2 != null)
                Number = x.Class2.SomeNumber;
            else if (x.Class3 != null)
                Number = x.Class3.SomeNumber;
            return Number == SomeNumber;
        })
    .ToList();

...however retValue doesn't contain any record.

The solution

Apparently I had to specify .Include statements because lazy loading was disabled and x.Class1, x.Class2 and x.Class3 always had the null value. I feel ashamed because I didn't explicitly state that lazy loading was disabled - the problem would have been obvious then.

However thanks to Ladislav's post, I improved my code like so:

Database DB = new Database(); // object context

int SomeNumber = 1; // some number
List<SomeClass> retValue = DB.SomeClass
    .Include("Class1")
    .Include("Class2")
    .Include("Class3")
    .Where(x =>
        SomeNumber == x.Class1.SomeNumber ||
        SomeNumber == x.Class2.SomeNumber ||
        SomeNumber == x.Class3.SomeNumber)
    .ToList();

I didn't know LINQ-to-Entities should perform automatic null coalescing.

IMHO you should be OK with just this:

Database DB = new Database(); 
var result = DB.SomeClass.Where(x =>
                            Number == x.Class1.SomeNumber ||
                            Number == x.Class2.SomeNumber ||
                            Number == x.Class3.SomeNumber)
                         .ToList();

Your query loads all data and after that you evaluate condition in .NET = you must test null value prior to accessing SomeNumber but that is not needed if you evaluate SomeNumber in SQL through Linq-to-entities. Linq-to-entities should perform automatic null coalescing.