实体框架 DbContext：值不能为空，参数名称来源

Question

I've started out using the CodeFirst approach in the entity framework.

When running my code I am unable to perform any operation on the DbSets within the DbContext - ProductsDb

I have checked the connection string and I think it is correct but attempting to perform operation results in the error

Value cannot be null, parameter source.

Here is the ProductsDb class

public class ProductsDb : DbContext
{
    public ProductsDb(string connectionString) : base(connectionString)
    {            
    }

    public ProductsDb()
    {            
    }

    public DbSet<AProduct> AProducts;
    public DbSet<BProduct> BProducts;
    public DbSet<CProduct> CProducts;
    public DbSet<DProduct> DProducts;
}

The connection string is defined in the app.config as follows:

<?xml version="1.0" encoding="utf-8" ?>
  <configuration>
    <connectionStrings>
    <add name="ProductsDb" providerName="System.Data.SqlClient" 
      connectionString="Data Source=MyPC\DEV;Initial Catalog=Test;User ID=sa;
      Password=pass;" />
    </connectionStrings>
  </configuration>

The code that throws the error is:

GetAProduct(string id)
{
   AProduct aProduct = null;

   using (ProductsDb productsDb = new ProductsDb())
   {
        aProduct = (from a in productsDb.AProducts
                    where a.Id== id
                    select a).FirstOrDefault();
   }

   return aProduct;
}

All of the product classes are plain old C# classes.

Any help would be really appreciated, I'm starting to pull my hair out. Never have any problems when writing Sql queries.

UPDATE: Copy Paste error the GetAProduct method has been altered.

Answer 1

You should define properties instead of fields for sets in your context class. So, instead of fields

public DbSet<AProduct> AProducts;

You should use properties

public DbSet<AProduct> AProducts { get; set; }

Because Entity Framework looks for properties. If you will explore sources of DbContext class, you will see, that in constructor in searches for declared set properties and initializes them:

private void DiscoverAndInitializeSets()
{
    new DbSetDiscoveryService(this).InitializeSets();
}

This set discover service looks only for properties:

var flags = BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance;
var properties = 
   from p in this._context.GetType().GetProperties(flags)
   where (p.GetIndexParameters().Length == 0) && 
         (p.DeclaringType != typeof(DbContext))
   select p);

foreach(PropertyInfo info in properties)
{
    // ...
}

If you will declare simple field of DbSet<T> type, EF will skip it's initialization, and field will have value null after context is created. Thats why where enumerator was throwing null source exception here:

productsDb.AProducts.Where(a => a.Id== id)

Answer 2

I had the same error message though the cause was different: I'd made all of the POCOs (and their properties) that were being used by the DbContext internal instead of public . As Sergey Berezovskiy has demonstrated in his answer, EF requires all class members to be public properties to initalise them. Changing them to public fixed the error straight away.

Answer 3

For future readers:

I have this issue trying to make a "read only" DbContext. (This specific project is all about "reports"). and wanting to expose only IQueryable (not DbSet).

My original (causing this issue) code looked like this:

    public IQueryable<OrganizationEntity> Organizations { get; set; }

    public IQueryable<DepartmentEntity> Departments { get; set; }

My altered code looked like this.

    internal DbSet<OrganizationEntity> InternalOrganizations { get; set; }

    internal DbSet<DepartmentEntity> InternalDepartments { get; set; }

    public IQueryable<OrganizationEntity> Organizations => this.InternalOrganizations.AsNoTracking();

    public IQueryable<DepartmentEntity> Departments => this.InternalApplicationDetails.AsNoTracking();

This is now working.