System.MissingMethodException：无法创建接口的实例

Question

I have a form that has a few text boxes, you input some values into the text boxes and then when you press submit it saves the values to a file. However when I press submit, I get the following exception. I've narrowed the problem down to something in the InventoryMngr and CreateInventory code, but I am unsure what I am doing wrong there.

System.MissingMethodException: Cannot create an instance of an interface.
   at HomeInventory2.Services.Factory.GetService(String servicename) in C:\Users\Protego\documents\visual studio 2010\Projects\HomeInventory2\HomeInventory2\Services\Factory.cs:line 37
   at HomeInventory2.Business.Manager.GetService(String name) in C:\Users\Protego\documents\visual studio 2010\Projects\HomeInventory2\HomeInventory2\Business\Manager.cs:line 14
   at HomeInventory2.Business.InventoryMngr.Create(CreateInventory inv) in C:\Users\Protego\documents\visual studio 2010\Projects\HomeInventory2\HomeInventory2\Business\InventoryMngr.cs:line 19
   at HomeInventory2.Form1.submitButton_Click(Object sender, EventArgs e) in C:\Users\Protego\documents\visual studio 2010\Projects\HomeInventory2\HomeInventory2\Form1.cs:line 52
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

InventoryMngr

namespace HomeInventory2.Business
{
    public class InventoryMngr : Manager
    {
        /// <summary>
        /// persists the inventory information
        /// </summary>
        /// <param name="inv"></param>
        public void Create(CreateInventory inv)
        {
            InventorySvc inventorySvc =
            (InventorySvc)GetService(typeof(InventorySvc).Name);
            inventorySvc.CreateInventory(inv);
        }
    }
}

CreateInventory

namespace HomeInventory2.Domain
{
    [Serializable]
    public class CreateInventory
    {


        /// <summary>
        /// item category
        /// </summary>
        private string itemCategory;
        public String ItemCategory
        {
            set
            {
                itemCategory = value;
            }
            get
            {
                return itemCategory;
            }
        }


        /// <summary>
        /// item properties
        /// </summary>
        private string itemProperties;
        public String ItemProperties
        {
            set
            {
                itemProperties = value;
            }
            get
            {
                return itemProperties;
            }
        }


        /// <summary>
        /// item amount
        /// </summary>
        private string itemAmount;
        public String ItemAmount
        {
            set
            {
                itemAmount = value;
            }
            get
            {
                return itemAmount;
            }
        }


        /// <summary>
        /// item value
        /// </summary>
        private string itemValue;
        public String ItemValue
        {
            set
            {
                itemValue = value;
            }
            get
            {
                return itemValue;
            }
        }

    }
}

InventorySvc is an interface

namespace HomeInventory2.Services
{
    public interface InventorySvc : IService
    {
        void CreateInventory(CreateInventory createinventory);
    }
}

InventoryImpl

namespace HomeInventory2.Services
{
    public class InventoryImpl: InventorySvc

    {
        /// <summary>
        /// Creates an output files with the given inventory information written to it, serves as placeholder - this will be replaced with a database system
        /// </summary>
        /// <param name="createinventory"></param>
        public void CreateInventory(CreateInventory createinventory)
        {
            try
            {
                FileStream fileStream = new FileStream
                ("CreateInventory.bin", FileMode.Create,
                FileAccess.Write);
                IFormatter formatter = new BinaryFormatter();
                formatter.Serialize(fileStream, createinventory);
                fileStream.Close();
            }
            catch (ItemNotFoundException)
            {
                throw new ItemNotFoundException("Output not created - see logs");
            }
        }
    }
}

Answer 1

我强烈怀疑您的GetService试图通过接口名称创建实例。在.Net中通过接口创建实例是非法的。

Answer 2

You cannot create an instance of an interface..

in your create method, cast your inventorySvc to class which implements InventorySvc

  InventorySvc inventorySvc = 
        (yourClassImplementingInventorySvcInterface)GetService(typeof(InventorySvc).Name); 

Answer 3

Interface cannot be instantiated. You should create a class that implements the interface.