c# - 同じプロパティであるオブジェクトを別のオブジェクトにマッピングする式

Question

Expressionこのコードを使用して単純なマッパーを作成しようとしています:

public static class MyUtility {

    public static Action<TSource, TTarget> BuildMapAction<TSource, TTarget>(IEnumerable<PropertyMap> properties) {

        var sourceInstance = Expression.Parameter(typeof(TSource), "source");
        var targetInstance = Expression.Parameter(typeof(TTarget), "target");

        var statements = BuildPropertyGettersSetters(sourceInstance, targetInstance, properties);

        Expression blockExp = Expression.Block(new[] { sourceInstance, targetInstance }, statements);

        if (blockExp.CanReduce)
            blockExp = blockExp.ReduceAndCheck();
        blockExp = blockExp.ReduceExtensions();

        var lambda = Expression.Lambda<Action<TSource, TTarget>>(blockExp, sourceInstance, targetInstance);

        return lambda.Compile();
    }

    private static IEnumerable<Expression> BuildPropertyGettersSetters(
        ParameterExpression sourceInstance,
        ParameterExpression targetInstance,
        IEnumerable<PropertyMap> properties) {

        var statements = new List<Expression>();

        foreach (var property in properties) {

            // value-getter
            var sourceGetterCall = Expression.Call(sourceInstance, property.SourceProperty.GetGetMethod());
            var sourcePropExp = Expression.TypeAs(sourceGetterCall, typeof(object));

            // value-setter
            var targetSetterCall =
                    Expression.Call(
                        targetInstance,
                        property.TargetProperty.GetSetMethod(),
                        Expression.Convert(sourceGetterCall, property.TargetProperty.PropertyType)
                        );
            var refNotNullExp = Expression.ReferenceNotEqual(sourceInstance, Expression.Constant(null));
            var propNotNullExp = Expression.ReferenceNotEqual(sourcePropExp, Expression.Constant(null));
            var notNullExp = Expression.And(refNotNullExp, propNotNullExp);
            var ifExp = Expression.IfThen(notNullExp, targetSetterCall);

            statements.Add(ifExp);
        }

        return statements;
    }

}

私にはすべて問題ないように思えますが、テストしようとすると、null 参照例外が発生します。テスト オブジェクトとメソッド:

public class UserEntity {

    public string Name { get; set; }
    public string Family { get; set; }
    public int Age { get; set; }
    public string Nickname { get; set; }

}

public class UserModel {

    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }
    public string Nickname { get; set; }

}

public static class CallTest {

    public static void Call() {
        var entity = new UserEntity {
            Name="Javad",
            Family="Amiry",
            Age = 25,
            Nickname = "my nickname is here",
        };
        var model = new UserModel();

        var map1 = new PropertyMap {
            SourceProperty = entity.GetType().GetProperty("Age"),
            TargetProperty = model.GetType().GetProperty("Age"),
        };
        var map2 = new PropertyMap {
            SourceProperty = entity.GetType().GetProperty("Nickname"),
            TargetProperty = model.GetType().GetProperty("Nickname"),
        };

        var action = MyUtility.BuildMapAction<UserEntity, UserModel>(new[] {map1, map2});
        action(entity, model); // here I get the error System.NullReferenceException: 'Object reference not set to an instance of an object.'
    }

}

そこで何が起こっているか分かりますか？私が逃したものは何ですか？

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注: サードパーティのマッパー (AutoMapper など) は使用できません