Expression.Convert：'System.Int64'类型的对象无法转换为'System.Int32'类型

Question

I asked a question yesterday here about reading properties from an anonymous object and writing them to private fields of a class. The problem solved. Here is the short story:

I have some data in json format. I deserialize them to ExpandoObject , and pass them as IDictionary<string, object> to method. It works fine, except Int32 properties. It seems they change to Int64 , where? I don't know.

Here is the method again:

    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();
    }

If we have this

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

class, and use this object

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

to initialize an instance of Person using above method, this error occurs:

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

But if we change Age property to:

public long Age { get; set; }

every thing looks fine and method works perfectly. I completely confused about why this happens. Do you have any idea?

Answer 1

So your input Dictionary contains long (based on discussion in comments).

The easiest fix is to add Convert.ChangeType before SetValue .
(passing in sourceValue and Constant(map.Field.FieldType) )
However, this may have an unintended consequence of allowing string -> int conversion.

Alternative is to add your own ConvertType method, where you decide how types are converted.

Answer 2

The expression is correct. The problem is Json.NET . It converts all numeric values (in anonymous conversions) to Int64 . So, I just need a custom convertor:

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();
    }
}

This is a concrete implementation, and absolutely can be implemented more extended and useful. But it just solve my current problem.