Reflection, properties and attributes - Studying ORM architecture

I'm just playing with attributes, which contains some info about a specific database field. I've already created the FieldAttribute class and it has default information, like name, type, etc.

My question is: is it possible to create a single method to retrieve this information and embbed into properties? I think I can communicate better with code:

[AttributeUsage(AttributeTargets.Property,AllowMultiple=false,Inherited=true)]
public class FieldAttribute:Attribute
{
    protected string _FieldName;
    protected esriFiel开发者_如何学PythondType _FieldType;
    protected bool _NotNullable;
    protected bool _IsPrimary;
    protected bool _IsForeign;
    protected int _Index;

    public virtual string FieldName
    {
        get { return this._FieldName; }
        set { this._FieldName = value; }
    }

    public virtual esriFieldType FieldType
    {
        get { return this._FieldType; }
        set { this._FieldType = value; }
    }

    public virtual bool NotNullable
    {
        get { return this._NotNullable; }
        set { this._NotNullable = value; }
    }

    public virtual int Index
    {
        get { return this._Index; }
        set { this._Index = value; }
    }

    public FieldAttribute(string fieldName, esriFieldType fieldType,int position)
    {
        this.FieldName = fieldName;
        this.FieldType = fieldType;
        _IsPrimary = false;
        _IsForeign = false;
        Index = position;
    }
}

// my main abstract class
public abstract class ObjectWrapper:IObjectWrapper
{
    protected int _ObjectId;
    protected IObject _UnderlyingObject;

    public virtual IObject UnderlyingObject
    {
        get { return this._UnderlyingObject; }
    }

    public virtual int ObjectId
    {
        get { return this._ObjectId; }
    }

    public virtual void Store()
    {
        try
        {
            _UnderlyingObject.Store();
        }
        catch (COMException comEx)
        {
            // log com exception
        }
        catch (Exception ex)
        {
            // log
        }
    }

    // this method retrieves field information
    private FieldAttribute GetFieldAttribute(string propertyName)
    {
        FieldAttribute propertyAttribute = null;

        try
        {
            PropertyInfo propInfo = this.GetType().GetProperty(propertyName);
            object[] attributes = propInfo.GetCustomAttributes(typeof(FieldAttribute), true);
            if (attributes.Length == 1)
                propertyAttribute = (FieldAttribute)attributes[0];
        }
        catch (AmbiguousMatchException ambEx)
        {
            // log
        }
        catch (ArgumentException argEx)
        {
            // log
        }

        return propertyAttribute;
    }
}

// my concrete class
public class Foo:ObjectWrapper
{        
    [Field("FOO_NAME",esriFieldType.esriFieldTypeString,1);
    public string FooName { get; set; }

    [Field("FOO_AGE",esriFieldType.esriFieldTypeInteger,2);
    public int FooAge { get; set; }
}

What I want to do here is to build a generic GetMethod, that uses Field attributes values to fetch the data from the database/_underlyingObject and just "fill out" the getter and setter for me.E.G:

when i call Foo.FooName, the getter will check for the attributes, passing the getter name to the GetFieldAttribute method.

How can I accomplish this?

The idea is that when there is a small framework, this will become a simple "ESRI database provider" for data.

I'm really sorry that I cannot explain this properly.

Thanks for all the help.

G.

---

Unless you're planning on instantiating your Foo-object through some Data Repository Retrieval method, this would require dynamic compilation to inject the necessary into your getter and setters.

I would instead recommend you to look at a pattern like this

var foo = DataRepository.GetObject<Foo>(some_id);

the method signature of GetObject would be

public static T GetObject<T>(object id) where T: new()

In the GetObject method you will reflect over the type finding all its Field-attributes and mapping them to a actual database call. This is very simple and common approach and can be achieved like this

public static IEnumerable<Tuple<PropertyInfo, FieldAttribute>> GetFieldAttributes<T>()
{
    var properties = typeof(T).GetProperties();
    foreach (var prop in properties)
    {
        var attribute = prop.GetCustomAttributes(typeof(FieldAttribute), true).FirstOrDefault();
        if (attribute != null)
        {
            yield return new Tuple<PropertyInfo, FieldAttribute>(prop, attribute);
        }
    }
}

Now with a list og attributes you can construct a sql-string like this

var columns = GetFieldAttributes<T>().Select(t => t.Item2.FieldName);
var sql = "SELECT "+ String.Join("," columns) +" FROM table WHERE id = "+ id;

var result = ... execute the query and return ie a datareader

With the result you can instantiate a Foo-instance, fill out its properties and return the instance

var instance = new T();

result.Read()
var dictionary = new Dictionary<string, object>();

// get the columns returned from database
for (int i = 0; i < reader.FieldCount; i++)
{
    string fieldName = reader.GetName(i);
    object value = reader.GetValue(i);

    dictionary.Add(fieldName, value);
}

// bind the columns from database to the properties of our object
foreach (var c in columns)
{
    var attr = c.Item2;
    var value = dictionary[attr.FieldName];

    if (value is DBNull)
    {
        value = null;
    }

    typeof(T).InvokeMember(c.Item1.Name, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.SetProperty | BindingFlags.Instance, null, instance, new[] { value });
}

// return the new object with all its values filled out
return instance