C# , using expression to get private static field value

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I have a base object TUserDataObject, and lots of subclasses ,UserInfo, CompanyInfo e.g. All classes have a private static field __id__. I will build an expression

C#

Func<int> GetId3<T>()

to get the field's value.
As you can see , if i want to get the field value of any subclass type, i should define a

C#

private Func<int> _idGetter = GetId3<UserInfo>();

If i have many subclasses , more than 2000 , maybe, it need define lots of getter.
Is it possible define a custom Func<TUserDataObject, int> to get any subclass's field value ?

What I have tried:

C#

public static Func<int> GetId3<T>()
{
    var t = typeof(T);
    var f = t.GetField("__id__", BindingFlags.GetField | BindingFlags.NonPublic | BindingFlags.Static);
    var fExp = Expression.Field(null, f);

    var valueExp = Expression.Convert(fExp, typeof(int));

    var lambda = Expression.Lambda<Func<int>>(valueExp);

    return lambda.Compile();
}

C#

private Func<int> _idGetter = <GetId3<UserInfo>();

C#

public class TUserDataObject
{
    private static int __id__ = -1;
}

public class UserInfo : TUserDataObject
{
    private static int __id__ = 1;
}

public class CompanyInfo : TUserDataObject
{
    private static int __id__ = 2;
}


public class UserModel : TUserDataObject
{
    private static int __id__ = 10000;
}

-----update----
Think about this ,

C#

private static readonly ConcurrentDictionary<TUserData, TableInfo> _tableInfos = new();

public bool TryGetTableInfo<TUserData>(out TableInfo<TUserData> tableInfo)
{
    var x = _tableInfos.GetOrAdd(typeOf(TUserData) , ... );
    tableInfo = x as TableInfo<TUserData>;
    ....
    return true;
}

but, we know that, UserMode <---mapping---> TableInfo are small amount， and we can use TableInfo[] instead of ConcurrentDictionary<>. with this scenes, the tableInfo's Index can be set in UserMode, for example, __id__.
so , i need a fast delegate call to get __id__ value in method TryGetTableInfo

public bool TryGetTableInfo<TUserData>(out TableInfo<TUserData> tableInfo)
{
    var x = _getter.Invoke(typeOf(TUserData));
    tableInfo = x as TableInfo<TUserData>;
    ....
    return true;
}

Posted yesterday

goldli88

Updated yesterday

v5

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2 solutions

Solution 1

You were close. Rather than return Func<int>, you can simply return int from your GetId method and extract the value inside. You're looking for something like this (I pulled this together in LinqPad):

C#

void Main()
{
	var x = new DoIt().GetId<CompanyInfo>();
	System.Diagnostics.Debug.WriteLine(x);
}

public class DoIt
{
  public int GetId<T>()
  {
    var t = typeof(T);
    var f = t.GetField("__id__", BindingFlags.GetField | BindingFlags.NonPublic | BindingFlags.Static);
    var fExp = Expression.Field(null, f);

    var valueExp = Expression.Convert(fExp, typeof(int));

    var lambda = Expression.Lambda<Func<int>>(valueExp);
	var value = lambda.Compile();
	return value();
  }
}

// You can define other methods, fields, classes and namespaces here
public class TUserDataObject
{
    private static int __id__ = -1;
}

public class UserInfo : TUserDataObject
{
    private static int __id__ = 1;
}

public class CompanyInfo : TUserDataObject
{
    private static int __id__ = 2;
}


public class UserModel : TUserDataObject
{
    private static int __id__ = 10000;
}

Posted yesterday

Pete O'Hanlon

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Richard Deeming · Accepted Answer · 2024-10-14T21:59:00

Quote:
Is it possible define a custom Func<TUserDataObject, int> to get any subclass's field value ?

You need to create an expression which will use reflection to get the field value. You're also going to need to pass an instance of your class to the function so that it can determine the correct type to access.

C#

public static Func<TUserDataObject, int> GetIdByInstance()
{
	var p = Expression.Parameter(typeof(TUserDataObject), "p");
	var type = Expression.Call(p, nameof(object.GetType), Array.Empty<Type>());
	var field = Expression.Call(type, nameof(Type.GetField), Array.Empty<Type>(), 
        Expression.Constant("__id__"), 
        Expression.Constant(BindingFlags.NonPublic | BindingFlags.Static));
    
	var fieldValue = Expression.Call(field, nameof(FieldInfo.GetValue), Array.Empty<Type>(), Expression.Constant(null));
	var body = Expression.Convert(fieldValue, typeof(int));
	var lambda = Expression.Lambda<Func<TUserDataObject, int>>(body, p);
	return lambda.Compile();
}

The lambda expression will be equivalent to:

C#

(TUserDataObject p) => (int)p.GetType().GetField("__id__", BindingFlags.NonPublic | BindingFlags.Static).GetValue(null);

Which then raises the question: why are you using LINQ expressions to build the method? You could just use the lambda method directly with the same effect:

C#

public static Func<TUserDataObject, int> GetIdByInstance()
    => (TUserDataObject p) => (int)p.GetType().GetField("__id__", BindingFlags.NonPublic | BindingFlags.Static).GetValue(null);

NB: You'll want to measure the performance impact of this. Reflection is notoriously slow, so if you're changing from a (hopefully cached) method per type to a single method that uses reflection, it will slow things down.

If you're using a relatively-recent version of .NET (7 or later), it may be better to use an interface with a static property to retrieve this information:
Explore static virtual members in C# interfaces | Microsoft Learn[^]