static class Program
    {
        static void Main(string[] args)
        {
            var a = new C1();
            Ext.SetValue(() => a.Msg, "TEST");
            Debug.Assert(a.Msg == "TEST");
        }
    }

    class C1
    {
        public string Msg { get; set; }
    }

    static class Ext
    {
        public static void SetValue<T>(Expression<Func<T>> expr, T value)
        {
            object obj;
            MemberInfo mInfo = expr.GetMemberInfo<T>(out obj);

            FieldInfo fInfo = mInfo as FieldInfo;
            PropertyInfo pInfo = mInfo as PropertyInfo;

            if (fInfo != null)
                fInfo.SetValue(obj, value);
            else if (pInfo != null && pInfo.CanWrite)
                pInfo.SetValue(obj, value, null);
        }

        public static MemberInfo GetMemberInfo<T>(this Expression<Func<T>> expr, out object obj)
        {
            if (expr == null)
                throw new ArgumentNullException("expr");

            var body = expr.Body as MemberExpression;

            if (body == null)
            {
                var unaryExpr = expr.Body as UnaryExpression;

                if (unaryExpr == null)
                    throw new ArgumentException("'expr' should be either an unary expression or a member expression");

                body = unaryExpr.Operand as MemberExpression;
            }

            if (body == null)
                throw new ArgumentException("'expr' should be a member expression");

            LambdaExpression lambdaExpr = Expression.Lambda(body.Expression);
            Delegate lambdaFunc = lambdaExpr.Compile();
            obj = lambdaFunc.DynamicInvoke();

            MemberInfo mInfo = obj.GetType().GetMember(body.Member.Name).FirstOrDefault();

            if (mInfo == null)
                throw new InvalidOperationException("Member not found.");

            return mInfo;
        }
    }