Question

Given:

FieldInfo field = <some valid string field on type T>; ParameterExpression targetExp = Expression.Parameter(typeof(T), 'target'); ParameterExpression valueExp = Expression.Parameter(typeof(string), 'value'); 

How do I compile a lambda expression to set the field on the ‘target’ parameter to ‘value’?

Answer 1

.Net 4.0 : now that there’s Expression.Assign, this is easy to do:

FieldInfo field = typeof(T).GetField('fieldName'); ParameterExpression targetExp = Expression.Parameter(typeof(T), 'target'); ParameterExpression valueExp = Expression.Parameter(typeof(string), 'value');  // Expression.Property can be used here as well MemberExpression fieldExp = Expression.Field(targetExp, field); BinaryExpression assignExp = Expression.Assign(fieldExp, valueExp);  var setter = Expression.Lambda<Action<T, string>>     (assignExp, targetExp, valueExp).Compile();  setter(subject, 'new value'); 

.Net 3.5 : you can’t, you’ll have to use System.Reflection.Emit instead:

class Program {     class MyObject     {         public int MyField;     }      static Action<T,TValue> MakeSetter<T,TValue>(FieldInfo field)     {         DynamicMethod m = new DynamicMethod(             'setter', typeof(void), new Type[] { typeof(T), typeof(TValue) }, typeof(Program));         ILGenerator cg = m.GetILGenerator();          // arg0.<field> = arg1         cg.Emit(OpCodes.Ldarg_0);         cg.Emit(OpCodes.Ldarg_1);         cg.Emit(OpCodes.Stfld, field);         cg.Emit(OpCodes.Ret);          return (Action<T,TValue>) m.CreateDelegate(typeof(Action<T,TValue>));     }      static void Main()     {         FieldInfo f = typeof(MyObject).GetField('MyField');          Action<MyObject,int> setter = MakeSetter<MyObject,int>(f);          var obj = new MyObject();         obj.MyField = 10;          setter(obj, 42);          Console.WriteLine(obj.MyField);         Console.ReadLine();     } }