LINQ 2实体：将int与字符串进行比较

Question

I need to construct a query to return a single entity based on a filter value. It must be executed on the server, LINQ 2 Objects is not an option.

The type of entity to return, the property to get it by and the value of that property are all only known at runtime and can be changed at runtime, so I need to do this as dynamically as possible. I feel like I'm almost there, but that final step keeps eluding me. Here's what I've tried so far:

Model containts the filter data

propParam will always be of type int

propModel will always be of type string

I can't change these types as this is just one of many filtering scenarios (a lookup).

private void SetFilter(Type typeToLookUp, string filterPropertyName)
{
    var propParam = typeToLookUp.GetProperty(filterPropertyName);
    var propModel = Model.GetType().GetProperty("FilterValue");

    var param = Expression.Parameter(typeToLookUp, "x");
    var bodyLeft = Expression.Property(param, propParam);
    var bodyRight = Expression.Property(Expression.Constant(Model), propModel);
    var body = Expression.Equal(bodyLeft, bodyRight);
    ...
}

This fails of course because I'm trying to compare an int (left) to a string (right).

So I need to convert the int to a string to be able to compare it, in LINQ 2 Entities SqlFunctions.StringConvert is used:

private void SetFilter(Type typeToLookUp, string filterPropertyName)
{
    var propParam = typeToLookUp.GetProperty(filterPropertyName);
    var propModel = Model.GetType().GetProperty("FilterValue");
    var stringConvertMethod = typeof(SqlFunctions).GetMethods(BindingFlags.Public | BindingFlags.Static)
                                .Single(
                                    x =>
                                    x.Name == "StringConvert" && x.GetParameters().Count() == 1 &&
                                    x.GetParameters()[0].ParameterType == typeof(double?));

    var param = Expression.Parameter(typeToLookUp, "x");
    var bodyLeft = Expression.Call(stringConvertMethod, Expression.Property(param, propParam));
    ...
}

Of course this method call will fail, because StringConvert doesn't take a parameter of type int. So I need to be able to cast the value of propParam (an integer) to double before I pass it to the StringConvert method. How would I go about doing that?

If I could just write the query it would look like this for example (this works):

var result = Repository.Query<Customer>().Where(x => SqlFunctions.StringConvert((double)x.Id) == Model.FilterValue);

Maybe it would be easier to just use Dynamic LINQ to do this, or write a raw SQL query, but I would like to know if there's a solution the way I've been trying first.

Answer 1

You can convert the int to a double in the expression by using Expression.Convert :

var param = Expression.Parameter(typeToLookUp, "x");
var property = Expression.Property(param, propParam);  
var doubleValue = Expression.Convert(property, typeof(double));
var bodyLeft = Expression.Call(stringConvertMethod, doubleValue);