using System;

using System.Linq.Expressions;

using System.Reflection;

​

public struct S

{

    public int I;

}

​

public class Program

{

    public static void Main()

    {

        object s = new S();

        typeof(S).GetField("I").SetValue(s, 1);

        Console.WriteLine(((S)s).I); // outputs "1"

        Action<object, object> setter = CreateSetter(typeof(S), typeof(S).GetField("I"));

        setter(s, 2);

        Console.WriteLine(((S)s).I); // outputs "2"

    }

    public static Action<object, object> CreateSetter(Type type, FieldInfo field)

    {

        //parameter "target", the object on which to set the field `field`

        ParameterExpression targetExp = Expression.Parameter(typeof(object), "target");

        //parameter "value" the value to be set in the `field` on "target"

        ParameterExpression valueExp = Expression.Parameter(typeof(object), "value");

        //cast the target from object to its correct type

        Expression castTartgetExp = type.IsValueType

            ? Expression.Unbox(targetExp, type)

            : Expression.Convert(targetExp, type);

        //cast the value to its correct type

        Expression castValueExp = Expression.Convert(valueExp, field.FieldType);

        //the field `field` on "target"

        MemberExpression fieldExp = Expression.Field(castTartgetExp, field);

        //MemberExpression fieldExp = Expression.Field(targetExp, field);

        //assign the "value" to the `field` 

        BinaryExpression assignExp = Expression.Assign(fieldExp, castValueExp);

        //BinaryExpression assignExp = Expression.Bind(field, )

        //compile the whole thing

        return Expression.Lambda<Action<object, object>>(assignExp, targetExp, valueExp).Compile();

    }

}