c# - 表达式.Convert : Object of type 'System.Int64' cannot be converted to type 'System.Int32'

Question

我昨天问了一个问题here关于从匿名对象读取属性并将它们写入类的私有(private)字段。问题解决了。这是一个小故事:

我有一些 json 格式的数据。我将它们反序列化为 ExpandoObject ，并将它们作为 IDictionary<string, object> 传递方法。它工作正常，除了Int32特性。似乎他们变成了Int64 ， 在哪里？我不知道。

这里还是方法:

    private Func<IDictionary<string, object>, dynamic> MakeCreator(
        Type type, Expression ctor,
        IEnumerable<PropertyToFieldMapper> maps) {

        var list = new List<Expression>();
        var vList = new List<ParameterExpression>();

        // creating new target
        var targetVariable = Expression.Variable(type, "targetVariable");
        vList.Add(targetVariable);
        list.Add(Expression.Assign(targetVariable, Expression.Convert(ctor, type)));

        // accessing source
        var sourceType = typeof(IDictionary<string, object>);
        var sourceParameter = Expression.Parameter(sourceType, "sourceParameter");

        // calling source ContainsKey(string) method
        var containsKeyMethodInfo = sourceType.GetMethod("ContainsKey", new[] { typeof(string) });

        var accessSourceIndexerProp = sourceType.GetProperty("Item");
        var accessSourceIndexerInfo = accessSourceIndexerProp.GetGetMethod();

        // itrate over writers and add their Call to block
        var containsKeyMethodArgument = Expression.Variable(typeof(string), "containsKeyMethodArgument");
        vList.Add(containsKeyMethodArgument);
        foreach (var map in maps) {
            list.Add(Expression.Assign(containsKeyMethodArgument, Expression.Constant(map.Property.Name)));
            var containsKeyMethodCall = Expression.Call(sourceParameter, containsKeyMethodInfo,
                                                        new Expression[] { containsKeyMethodArgument });

            // creating writer
            var sourceValue = Expression.Call(sourceParameter, accessSourceIndexerInfo,
                                              new Expression[] { containsKeyMethodArgument });
            var setterInfo = map.Field.GetType().GetMethod("SetValue", new[] { typeof(object), typeof(object) });
            var setterCall = Expression.Call(Expression.Constant(map.Field), setterInfo,
                new Expression[] {
                                     Expression.Convert(targetVariable, typeof(object)),
                                     Expression.Convert(sourceValue, typeof(object))
                                 });
            Console.WriteLine(Expression.Lambda(setterCall));
            list.Add(Expression.IfThen(containsKeyMethodCall, setterCall));
        }
        list.Add(targetVariable);

        var block = Expression.Block(vList, list);

        var lambda = Expression.Lambda<Func<IDictionary<string, object>, dynamic>>(
            block, new[] { sourceParameter }
            );

        return lambda.Compile();
    }

如果我们有这个

public class Person {
    public int Age { get; set; }
    public string Name { get; set; }
}

类，并使用这个对象

var data = new { Name = "Amiry", Age = 20 };

初始化Person的实例使用上述方法，出现此错误:

Object of type 'System.Int64' cannot be converted to type 'System.Int32'.

但是如果我们改变Age属性:

public long Age { get; set; }

一切看起来都很好，方法也很完美。我完全不明白为什么会这样。你有什么想法吗？

Answer 1

表述正确。问题是 Json.NET。它将所有数值(在匿名转换中)转换为 Int64。所以，我只需要一个自定义转换器:

public class JsonIntegerConverter : JsonConverter {

    public override bool CanConvert(Type objectType) {
        return objectType == typeof(IDictionary<string, object>);
    }

    public override bool CanWrite {
        get { return false; }
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {
        var result = new Dictionary<string, object>();
        reader.Read();

        while (reader.TokenType == JsonToken.PropertyName) {
            var propertyName = (string)reader.Value;
            reader.Read();
            object value;
            if (reader.TokenType == JsonToken.Integer) {
                var temp = Convert.ToInt64(reader.Value);
                if (temp <= Byte.MaxValue && temp >= Byte.MinValue)
                    value = Convert.ToByte(reader.Value);
                else if (temp >= Int16.MinValue && temp <= Int16.MaxValue)
                    value = Convert.ToInt16(reader.Value);
                else if (temp >= Int32.MinValue && temp <= Int32.MaxValue)
                    value = Convert.ToInt32(reader.Value);
                else
                    value = temp;
            } else
                value = serializer.Deserialize(reader);
            result.Add(propertyName, value);
            reader.Read();
        }

        return result;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
        throw new NotSupportedException();
    }
}

这是一个具体的实现，绝对可以实现得更加扩展和有用。但它只是解决了我目前的问题。