How to dynamically create a class?

It will take some work, but is certainly not impossible.

What I have done is:

• Create a C# source in a string (no need to write out to a file),
• Run it through the Microsoft.CSharp.CSharpCodeProvider (CompileAssemblyFromSource)
• Find the generated Type
• And create an instance of that Type (Activator.CreateInstance)

This way you can deal with the C# code you already know, instead of having to emit MSIL.

But this works best if your class implements some interface (or is derived from some baseclass), else how is the calling code (read: compiler) to know about that class that will be generated at runtime?

You can also dynamically create a class by using DynamicObject.

public class DynamicClass : DynamicObject
{
    private Dictionary<string, KeyValuePair<Type, object>> _fields;

    public DynamicClass(List<Field> fields)
    {
        _fields = new Dictionary<string, KeyValuePair<Type, object>>();
        fields.ForEach(x => _fields.Add(x.FieldName,
            new KeyValuePair<Type, object>(x.FieldType, null)));
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        if (_fields.ContainsKey(binder.Name))
        {
            var type = _fields[binder.Name].Key;
            if (value.GetType() == type)
            {
                _fields[binder.Name] = new KeyValuePair<Type, object>(type, value);
                return true;
            }
            else throw new Exception("Value " + value + " is not of type " + type.Name);
        }
        return false;
    }

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        result = _fields[binder.Name].Value;
        return true;
    }
}

I store all class fields in a dictionary _fields together with their types and values. The both methods are to can get or set value to some of the properties. You must use the dynamic keyword to create an instance of this class.

The usage with your example:

var fields = new List<Field>() { 
    new Field("EmployeeID", typeof(int)),
    new Field("EmployeeName", typeof(string)),
    new Field("Designation", typeof(string)) 
};

dynamic obj = new DynamicClass(fields);

//set
obj.EmployeeID = 123456;
obj.EmployeeName = "John";
obj.Designation = "Tech Lead";

obj.Age = 25;             //Exception: DynamicClass does not contain a definition for 'Age'
obj.EmployeeName = 666;   //Exception: Value 666 is not of type String

//get
Console.WriteLine(obj.EmployeeID);     //123456
Console.WriteLine(obj.EmployeeName);   //John
Console.WriteLine(obj.Designation);    //Tech Lead

Edit: And here is how looks my class Field:

public class Field
{
    public Field(string name, Type type)
    {
        this.FieldName = name;
        this.FieldType = type;
    }

    public string FieldName;

    public Type FieldType;
}

Yes, you can use System.Reflection.Emit namespace for this. It is not straight forward if you have no experience with it, but it is certainly possible.

Edit: This code might be flawed, but it will give you the general idea and hopefully off to a good start towards the goal.

using System;
using System.Reflection;
using System.Reflection.Emit;

namespace TypeBuilderNamespace
{
    public static class MyTypeBuilder
    {
        public static void CreateNewObject()
        {
            var myType = CompileResultType();
            var myObject = Activator.CreateInstance(myType);
        }
        public static Type CompileResultType()
        {
            TypeBuilder tb = GetTypeBuilder();
            ConstructorBuilder constructor = tb.DefineDefaultConstructor(MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.RTSpecialName);

            // NOTE: assuming your list contains Field objects with fields FieldName(string) and FieldType(Type)
            foreach (var field in yourListOfFields)
                CreateProperty(tb, field.FieldName, field.FieldType);

            Type objectType = tb.CreateType();
            return objectType;
        }

        private static TypeBuilder GetTypeBuilder()
        {
            var typeSignature = "MyDynamicType";
            var an = new AssemblyName(typeSignature);
            AssemblyBuilder assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(an, AssemblyBuilderAccess.Run);
            ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("MainModule");
            TypeBuilder tb = moduleBuilder.DefineType(typeSignature,
                    TypeAttributes.Public |
                    TypeAttributes.Class |
                    TypeAttributes.AutoClass |
                    TypeAttributes.AnsiClass |
                    TypeAttributes.BeforeFieldInit |
                    TypeAttributes.AutoLayout,
                    null);
            return tb;
        }

        private static void CreateProperty(TypeBuilder tb, string propertyName, Type propertyType)
        {
            FieldBuilder fieldBuilder = tb.DefineField("_" + propertyName, propertyType, FieldAttributes.Private);

            PropertyBuilder propertyBuilder = tb.DefineProperty(propertyName, PropertyAttributes.HasDefault, propertyType, null);
            MethodBuilder getPropMthdBldr = tb.DefineMethod("get_" + propertyName, MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.HideBySig, propertyType, Type.EmptyTypes);
            ILGenerator getIl = getPropMthdBldr.GetILGenerator();

            getIl.Emit(OpCodes.Ldarg_0);
            getIl.Emit(OpCodes.Ldfld, fieldBuilder);
            getIl.Emit(OpCodes.Ret);

            MethodBuilder setPropMthdBldr =
                tb.DefineMethod("set_" + propertyName,
                  MethodAttributes.Public |
                  MethodAttributes.SpecialName |
                  MethodAttributes.HideBySig,
                  null, new[] { propertyType });

            ILGenerator setIl = setPropMthdBldr.GetILGenerator();
            Label modifyProperty = setIl.DefineLabel();
            Label exitSet = setIl.DefineLabel();

            setIl.MarkLabel(modifyProperty);
            setIl.Emit(OpCodes.Ldarg_0);
            setIl.Emit(OpCodes.Ldarg_1);
            setIl.Emit(OpCodes.Stfld, fieldBuilder);

            setIl.Emit(OpCodes.Nop);
            setIl.MarkLabel(exitSet);
            setIl.Emit(OpCodes.Ret);

            propertyBuilder.SetGetMethod(getPropMthdBldr);
            propertyBuilder.SetSetMethod(setPropMthdBldr);
        }
    }
}

I know i reopen this old task but with c# 4.0 this task is absolutely painless.

dynamic expando = new ExpandoObject();
expando.EmployeeID=42;
expando.Designation="unknown";
expando.EmployeeName="curt"

//or more dynamic
AddProperty(expando, "Language", "English");

for more see https://www.oreilly.com/learning/building-c-objects-dynamically