Drag and drop is a convenient and intuitive method of exchanging data between windows (and applications).  ESRI applications like ArcCatalog and ArcMap have drop and drop capabilities, for example, you can drag layer into the Buffer geoprocessing tool window (see below). 

Using the sample code below you can add this capability to your .NET application, that is, you can make your window/control a drop target for layers dragged from the ArcMap table of contents.  This code can be used in window hosted by an ESRI application such as an ArcMap dockable widnow or as a standalone application.

To enable this feature a utility class called "EsriDataObject" was developed to deserialize the dropped object from ArcMap to a collection of ESRI layers.  The source code to "EsriDataObject" is provided in the second code sample below.

The first code sample demonstrates how to use "EsriDataObject" in a .NET form.  The code shows a Form called "form1" with a TextBox called "textBox1".  The TextBox will display the name(s) of layers that are dropped from ArcMap. 

Sample 1: How to use "EsriDataObject" in a .NET Form.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

using ESRI.ArcGIS.Carto;

namespace MyNamespace {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
            this.textBox1.AllowDrop = true;
            this.textBox1.DragEnter += new DragEventHandler(this.textBox1_DragEnter);
            this.textBox1.DragOver += new DragEventHandler(this.textBox1_DragOver);
            this.textBox1.DragDrop += new DragEventHandler(this.textBox1_DragDrop);
        }
        private void textBox1_DragEnter(object sender, DragEventArgs e) {
            e.Effect = EsriDataObject.IsValid(e.Data) ? DragDropEffects.All : DragDropEffects.None;
        }
        private void textBox1_DragOver(object sender, DragEventArgs e) {
            e.Effect = EsriDataObject.IsValid(e.Data) ? DragDropEffects.All : DragDropEffects.None;
        }
        private void textBox1_DragDrop(object sender, DragEventArgs e) {
            EsriDataObject esriDataObject = EsriDataObject.ConvertToEsriDataObject(e.Data);
            foreach (ILayer layer in esriDataObject.LayerCollection) {
                this.textBox1.Text += layer.Name + " ";
            }
        }
    }
}

Sample 2: Source code to "EsriDataObject".

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Windows.Forms;

using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.ArcMapUI;
using ESRI.ArcGIS.esriSystem;

namespace MyNamespace {
    public class EsriDataObject {
        private const string DATAOBJECT_ESRILAYERS = "ESRI Layers";
        private const string INTERFACE_ILAYER = "{34C20002-4D3C-11D0-92D8-00805F7C28B0}";
        private const string INTERFACE_ITABLEPROPERTY = "{4657D951-5FFB-11D3-9F6C-00C04F6BC886}";
        private readonly List<ILayer> m_layerCollection = null;
        private readonly List<ITableProperty> m_tablePropertyCollection = null;
        //
        // CONSTRUCTOR
        //
        public EsriDataObject() {
            this.m_layerCollection = new List<ILayer>();
            this.m_tablePropertyCollection = new List<ITableProperty>();
        }
        //
        // PROPERTIES
        //
        public List<ILayer> LayerCollection {
            get { return this.m_layerCollection; }
        }
        public List<ITableProperty> TablePropertyCollection {
            get { return this.m_tablePropertyCollection; }
        }
        //
        // STATIC METHODS
        //
        public static bool IsValid(IDataObject dataObject) {
            return dataObject.GetDataPresent(DATAOBJECT_ESRILAYERS);
        }
        public static EsriDataObject ConvertToEsriDataObject(IDataObject dataObject) {
            //
            EsriDataObject esriDataObject = new EsriDataObject();

            // Exit if dropped object is invalid
            if (EsriDataObject.IsValid(dataObject)) {
                // Get Byte Array from DataObject
                object esriLayers = dataObject.GetData(DATAOBJECT_ESRILAYERS);
                MemoryStream memoryStream = (MemoryStream)esriLayers;
                byte[] bytes = memoryStream.ToArray();

                // Load Byte Array into a Stream (ESRI Wrapper of IStream)
                IMemoryBlobStreamVariant memoryBlobStreamVariant = new MemoryBlobStreamClass();
                memoryBlobStreamVariant.ImportFromVariant(bytes);
                IMemoryBlobStream2 memoryBlobStream = (IMemoryBlobStream2)memoryBlobStreamVariant;
                IStream stream = (IStream)memoryBlobStream;

                // Load Stream into an ESRI ObjectStream
                IObjectStream objectStream = new ObjectStreamClass();
                objectStream.Stream = stream;

                // Get Number of Layers in Dropped Object
                byte pv;
                uint cb = sizeof(int);
                uint pcbRead;
                objectStream.RemoteRead(out pv, cb, out pcbRead);
                int count = Convert.ToInt32(pv);

                // Define Guids
                Guid guidLayer = new Guid(INTERFACE_ILAYER);
                Guid guidTable = new Guid(INTERFACE_ITABLEPROPERTY);

                // Get Dropped Layers
                for (int i = 0; i < count; i++) {
                    object o = objectStream.LoadObject(ref guidLayer, null);
                    ILayer layer = (ILayer)o;
                    esriDataObject.LayerCollection.Add(layer);
                }

                // Get Dropped TableProperties
                for (int i = 0; i < count; i++) {
                    object o = objectStream.LoadObject(ref guidTable, null);
                    if (o == null) { continue; }
                    ITableProperty tableProperty = (ITableProperty)o;
                    esriDataObject.TablePropertyCollection.Add(tableProperty);
                }
            }

            return esriDataObject;
        }
    }
}