Custom attributes in c# for data types

Question

I am thinking about making a custom attribute so that when we are using multiple data readers [SqldataReader] on different objects/tables, we could use the attribute to get the type of the property, and the "columnName" of the property. This way, we could then have a method that takes the data reader as a param, and from there could reflect the attributes to read in the columns. An example of what is currently being done is below, and then an example of what I am trying to accomplish. The problem I am having, is how to manage how to tell it what the (Type) is.

 private static App GetAppInfo(SqlDataReader dr)
    {
        App app = new App();

        app.ID = MCCDBUtility.GetDBValueInt(dr, "APPLICATION_ID");
        app.Name = MCCDBUtility.GetDBValueString(dr, "APPNAME");
        app.Desc = MCCDBUtility.GetDBValueString(dr, "APPDESCRIPTION");
        app.Version = MCCDBUtility.GetDBValueString(dr, "APP_VERSION");
        app.Type = MCCDBUtility.GetDBValueString(dr, "APPLICATIONTYPEID");
        app.AreaName = MCCDBUtility.GetDBValueString(dr, "AREANAME");

        return app;
    }

What I am thinking though, so if I had a class for example like so:

[DataReaderHelper("MethodNameToGetType", "ColumnName")]
public string APPNAME {get;set;}

How could I go about this?

Answer 1

Fist of all, this is possible and if you like I could add a code sample. But: This is not a good idea.

Why, you ask?

First - DataReader provides you with a method GetSchemaTable() which contains a property DataType which is a System.Type object. So basically you could create a MCCDBUtility.GetValue(dr, "columnName") that does the logic for your.

Second - What about you have a int property on your object but your datareader returns a decimal. For that case you can use Convert.ChangeType(value, type)

If you combine that you can achive what you want with

instance.Id = MCCDBUtility.GetValue<int>(dr, "columnName")

public T GetValue<T>(IDataReader reader, string columnName)
{
     object value GetValue(reader, columnName);
     return Convert.ChangeType(value, typeof(T));
}

private object GetValue(IDataReader reader, string columnName)
{
    var schmema = reader.GetSchemaTable();
    var dbType = typeof(object);
    foreach(DataRowView row in schema.DefaultView)
        if (row["columnName"].ToString().Equals(columnName, StringComparer.OrdinalIgnoreCase))
            return row["ColumnType"];

    if (dbType.Equals(typeof(int))
        return GetInt(reader, columnName)
    ... // you get the point
    else
        return GetObject(reader, columnName);
}

And Third - Don't do this anyway there are great tools for mapping your query to your business objects. I don't want to name them all but a very lightweight and easy to understand is Dapper.NET, give it a try. https://github.com/StackExchange/dapper-dot-net

In combination with https://github.com/tmsmith/Dapper-Extensions you can easily map your database queries to your pocos

Update

As promised, here is the code for implementing on your own. Just create a Visual Studio Test project, insert the code and let it run. For readablity I omitted the unused IReadReader interface implementations, so you have to let intellisense create them for you.

Run the test and enjoy.

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        var values = new Dictionary<string, object>();
        values.Add("ProductId", 17);
        values.Add("ProductName", "Something");
        values.Add("Price", 29.99M);

        var reader = new FakeDataReader(values);

        var product1 = new Product();
        reader.SetValue(product1, p => p.Id);
        reader.SetValue(product1, p => p.Name);
        reader.SetValue(product1, p => p.Price);

        Assert.AreEqual(17, product1.Id);
        Assert.AreEqual("Something", product1.Name);
        Assert.AreEqual(29.99M, product1.Price);

        var product2 = new Product();
        reader.SetAllValues(product2);

        Assert.AreEqual(17, product2.Id);
        Assert.AreEqual("Something", product2.Name);
        Assert.AreEqual(29.99M, product2.Price);
    }

}

public class Product
{
    [Mapping("ProductId")]
    public int Id { get; set; }

    [Mapping("ProductName")]
    public string Name { get; set; }

    public decimal Price { get; set; }
}

[AttributeUsage(AttributeTargets.Property, AllowMultiple=false)]
public class MappingAttribute : Attribute
{
    public MappingAttribute(string columnName)
    {
        this.ColumnName = columnName;
    }

    public string ColumnName { get; private set; }
}

public static class IDataReaderExtensions
{
    public static void SetAllValues(this IDataReader reader, object source)
    {
        foreach (var prop in source.GetType().GetProperties())
        {
            SetValue(reader, source, prop);
        }
    }
    public static void SetValue<T, P>(this IDataReader reader, T source, Expression<Func<T, P>> pe)
    {
        var property = (PropertyInfo)((MemberExpression)pe.Body).Member;
        SetValue(reader, source, property);
    }

    private static void SetValue(IDataReader reader, object source, PropertyInfo property)
    {
        string propertyName = property.Name;
        var columnName = propertyName;
        var mapping = property.GetAttribute<MappingAttribute>();
        if (mapping != null) columnName = mapping.ColumnName;

        var value = reader.GetValue(reader.GetOrdinal(columnName));
        var value2 = Convert.ChangeType(value, property.PropertyType);
        property.SetValue(source, value2, null);
    }
}

public static class ICustomFormatProviderExtensions
{
    public static T GetAttribute<T>(this ICustomAttributeProvider provider)
    {
        return (T)provider.GetCustomAttributes(typeof(T), true).FirstOrDefault();
    }
}

public class FakeDataReader : IDataReader
{
    private Dictionary<string, object> values;

    public FakeDataReader(Dictionary<string, object> values)
    {
        this.values = values;
    }

    public int GetOrdinal(string name)
    {
        int i = 0;
        foreach (var key in values.Keys)
        {
            if (key.Equals(name, StringComparison.OrdinalIgnoreCase)) return i;
            i++;
        }
        return -1;
    }

    public object GetValue(int i)
    {
        return values.Values.ToArray()[i];
    }
}