How to dynamically create a class?

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

As Hans suggested, you can use Roslyn to dynamically create classes.

Full source:

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Reflection;

namespace RoslynDemo1
{
    class Program
    {
        static void Main(string[] args)
        {
            var fields = new List<Field>()
            {
                new Field("EmployeeID","int"),
                new Field("EmployeeName","String"),
                new Field("Designation","String")
            };

            var employeeClass = CreateClass(fields, "Employee");

            dynamic employee1 = Activator.CreateInstance(employeeClass);
            employee1.EmployeeID = 4213;
            employee1.EmployeeName = "Wendy Tailor";
            employee1.Designation = "Engineering Manager";

            dynamic employee2 = Activator.CreateInstance(employeeClass);
            employee2.EmployeeID = 3510;
            employee2.EmployeeName = "John Gibson";
            employee2.Designation = "Software Engineer";

            Console.WriteLine($"{employee1.EmployeeName}");
            Console.WriteLine($"{employee2.EmployeeName}");

            Console.WriteLine("Press any key to continue...");
            Console.ReadKey();
        }

        public static Type CreateClass(List<Field> fields, string newClassName, string newNamespace = "Magic")
        {
            var fieldsCode = fields
                                .Select(field => $"public {field.FieldType} {field.FieldName};")
                                .ToString(Environment.NewLine);

            var classCode = $@"
                using System;

                namespace {newNamespace}
                {{
                    public class {newClassName}
                    {{
                        public {newClassName}()
                        {{
                        }}

                        {fieldsCode}
                    }}
                }}
            ".Trim();

            classCode = FormatUsingRoslyn(classCode);


            var assemblies = new[]
            {
                MetadataReference.CreateFromFile(typeof(object).Assembly.Location),
            };

            /*
            var assemblies = AppDomain
                        .CurrentDomain
                        .GetAssemblies()
                        .Where(a => !string.IsNullOrEmpty(a.Location))
                        .Select(a => MetadataReference.CreateFromFile(a.Location))
                        .ToArray();
            */

            var syntaxTree = CSharpSyntaxTree.ParseText(classCode);

            var compilation = CSharpCompilation
                                .Create(newNamespace)
                                .AddSyntaxTrees(syntaxTree)
                                .AddReferences(assemblies)
                                .WithOptions(new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));

            using (var ms = new MemoryStream())
            {
                var result = compilation.Emit(ms);
                //compilation.Emit($"C:\\Temp\\{newNamespace}.dll");

                if (result.Success)
                {
                    ms.Seek(0, SeekOrigin.Begin);
                    Assembly assembly = Assembly.Load(ms.ToArray());

                    var newTypeFullName = $"{newNamespace}.{newClassName}";

                    var type = assembly.GetType(newTypeFullName);
                    return type;
                }
                else
                {
                    IEnumerable<Diagnostic> failures = result.Diagnostics.Where(diagnostic =>
                        diagnostic.IsWarningAsError ||
                        diagnostic.Severity == DiagnosticSeverity.Error);

                    foreach (Diagnostic diagnostic in failures)
                    {
                        Console.Error.WriteLine("{0}: {1}", diagnostic.Id, diagnostic.GetMessage());
                    }

                    return null;
                }
            }
        }

        public static string FormatUsingRoslyn(string csCode)
        {
            var tree = CSharpSyntaxTree.ParseText(csCode);
            var root = tree.GetRoot().NormalizeWhitespace();
            var result = root.ToFullString();
            return result;
        }
    }

    public class Field
    {
        public string FieldName;
        public string FieldType;

        public Field(string fieldName, string fieldType)
        {
            FieldName = fieldName;
            FieldType = fieldType;
        }
    }

    public static class Extensions
    {
        public static string ToString(this IEnumerable<string> list, string separator)
        {
            string result = string.Join(separator, list);
            return result;
        }
    }
}

You can also dynamically create a class by using DynamicExpressions.

Since 'Dictionary's have compact initializers and handle key collisions, you will want to do something like this.

  var list = new Dictionary<string, string> {
    {
      "EmployeeID",
      "int"
    }, {
      "EmployeeName",
      "String"
    }, {
      "Birthday",
      "DateTime"
    }
  };

Or you might want to use a JSON converter to construct your serialized string object into something manageable.

Then using System.Linq.Dynamic;

  IEnumerable<DynamicProperty> props = list.Select(property => new DynamicProperty(property.Key, Type.GetType(property.Value))).ToList();

  Type t = DynamicExpression.CreateClass(props);

The rest is just using System.Reflection.

  object obj = Activator.CreateInstance(t);
  t.GetProperty("EmployeeID").SetValue(obj, 34, null);
  t.GetProperty("EmployeeName").SetValue(obj, "Albert", null);
  t.GetProperty("Birthday").SetValue(obj, new DateTime(1976, 3, 14), null);
}  

For those wanting to create a dynamic class just properties (i.e. POCO), and create a list of this class. Using the code provided later, this will create a dynamic class and create a list of this.

var properties = new List<DynamicTypeProperty>()
{
    new DynamicTypeProperty("doubleProperty", typeof(double)),
    new DynamicTypeProperty("stringProperty", typeof(string))
};

// create the new type
var dynamicType = DynamicType.CreateDynamicType(properties);
// create a list of the new type
var dynamicList = DynamicType.CreateDynamicList(dynamicType);

// get an action that will add to the list
var addAction = DynamicType.GetAddAction(dynamicList);

// call the action, with an object[] containing parameters in exact order added
addAction.Invoke(new object[] {1.1, "item1"});
addAction.Invoke(new object[] {2.1, "item2"});
addAction.Invoke(new object[] {3.1, "item3"});

Here are the classes that the previous code uses.

Note: You'll also need to reference the Microsoft.CodeAnalysis.CSharp library.

       /// <summary>
    /// A property name, and type used to generate a property in the dynamic class.
    /// </summary>
    public class DynamicTypeProperty
    {
        public DynamicTypeProperty(string name, Type type)
        {
            Name = name;
            Type = type;
        }
        public string Name { get; set; }
        public Type Type { get; set; }
    }

   public static class DynamicType
    {
        /// <summary>
        /// Creates a list of the specified type
        /// </summary>
        /// <param name="type"></param>
        /// <returns></returns>
        public static IEnumerable<object> CreateDynamicList(Type type)
        {
            var listType = typeof(List<>);
            var dynamicListType = listType.MakeGenericType(type);
            return (IEnumerable<object>) Activator.CreateInstance(dynamicListType);
        }

        /// <summary>
        /// creates an action which can be used to add items to the list
        /// </summary>
        /// <param name="listType"></param>
        /// <returns></returns>
        public static Action<object[]> GetAddAction(IEnumerable<object> list)
        {
            var listType = list.GetType();
            var addMethod = listType.GetMethod("Add");
            var itemType = listType.GenericTypeArguments[0];
            var itemProperties = itemType.GetProperties();

            var action = new Action<object[]>((values) =>
            {
                var item = Activator.CreateInstance(itemType);

                for(var i = 0; i < values.Length; i++)
                {
                    itemProperties[i].SetValue(item, values[i]);
                }

                addMethod.Invoke(list, new []{item});
            });

            return action;
        }

        /// <summary>
        /// Creates a type based on the property/type values specified in the properties
        /// </summary>
        /// <param name="properties"></param>
        /// <returns></returns>
        /// <exception cref="Exception"></exception>
        public static Type CreateDynamicType(IEnumerable<DynamicTypeProperty> properties)
        {
            StringBuilder classCode = new StringBuilder();

            // Generate the class code
            classCode.AppendLine("using System;");
            classCode.AppendLine("namespace Dexih {");
            classCode.AppendLine("public class DynamicClass {");

            foreach (var property in properties)
            {
                classCode.AppendLine($"public {property.Type.Name} {property.Name} {{get; set; }}");
            }
            classCode.AppendLine("}");
            classCode.AppendLine("}");

            var syntaxTree = CSharpSyntaxTree.ParseText(classCode.ToString());

            var references = new MetadataReference[]
            {
                MetadataReference.CreateFromFile(typeof(object).GetTypeInfo().Assembly.Location),
                MetadataReference.CreateFromFile(typeof(DictionaryBase).GetTypeInfo().Assembly.Location)
            };

            var compilation = CSharpCompilation.Create("DynamicClass" + Guid.NewGuid() + ".dll",
                syntaxTrees: new[] {syntaxTree},
                references: references,
                options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));

            using (var ms = new MemoryStream())
            {
                var result = compilation.Emit(ms);

                if (!result.Success)
                {
                    var failures = result.Diagnostics.Where(diagnostic =>
                        diagnostic.IsWarningAsError ||
                        diagnostic.Severity == DiagnosticSeverity.Error);

                    var message = new StringBuilder();

                    foreach (var diagnostic in failures)
                    {
                        message.AppendFormat("{0}: {1}", diagnostic.Id, diagnostic.GetMessage());
                    }

                    throw new Exception($"Invalid property definition: {message}.");
                }
                else
                {

                    ms.Seek(0, SeekOrigin.Begin);
                    var assembly = System.Runtime.Loader.AssemblyLoadContext.Default.LoadFromStream(ms);
                    var dynamicType = assembly.GetType("Dexih.DynamicClass");
                    return dynamicType;
                }
            }
        }
    }

you can use CSharpProvider:

var code = @"
    public class Abc {
       public string Get() { return ""abc""; }
    }
";

var options = new CompilerParameters();
options.GenerateExecutable = false;
options.GenerateInMemory = false;

var provider = new CSharpCodeProvider();
var compile = provider.CompileAssemblyFromSource(options, code);

var type = compile.CompiledAssembly.GetType("Abc");
var abc = Activator.CreateInstance(type);

var method = type.GetMethod("Get");
var result = method.Invoke(abc, null);

Console.WriteLine(result); //output: abc