Convert my value type to nullable equivalent

问题

I have an ad-hoc reporting system; I have no compile-time knowledge of the source type of queries or of the required fields. I could write expression trees at runtime using the System.Linq.Expressions.Expression factory methods, and invoke LINQ methods using reflection, but Dynamic LINQ is a simpler solution.

The reporting system is to allow for queries which returns the result of a LEFT JOIN. There are fields in the joined table which are NOT NULL in the database; but because this is a LEFT JOIN, those fields will contain NULL for certain records. The EF6-generated expression falls on this, because the expression projects to a non-nullable value type.

If I was doing this in compile-time LINQ, I would explicitly cast to the nullable type:

enum Color { Red, Green,  Blue }

// using System;
// using static System.Linq.Enumerable;
// using System.Linq;

var range = Range(0, 3).Select(x => (Color)x).AsQueryable();
var qry = range.Select(x => (Color?)x);

Dynamic LINQ supports explicit conversions:

// using static System.Linq.Dynamic.Core

var qry1 = range.Select("int?(it)");

but only a specific set of types can be referenced in the query. If I try to use Color in the query:

var qry2 = range.Select("Color(it)");

I get the following error:

No applicable method 'Color' exists in type 'Color'

and if I try to explicitly cast to Color?:

var qry3 = range.Select("Color?(it)");

I get:

Requested value 'Color' was not found.

How can I do this using the Dynamic LINQ library?

回答1:

Dynamic LINQ provides a Cast method which can be used as follows:

var range = Enumerable.Range(0,3).Select(x => (Color)x).AsQueryable();
var castDynamic = range.Cast(typeof(Color?)).ToDynamicArray();
castDynamic.Dump();

You can also pass a string with the name of the output type. Note that for nullable types, you need the full name of the type:

string s = typeof(Color?).FullName;
s.Dump();
var castDynamicFromString = range.Cast(s);
castDynamicFromString.Dump();

Cast can also be used within the Dynamic LINQ string expression, either passing in a Type object as a parameter, or by using the name directly:

var castInSelect = range.Select($@"Cast(""{s}""").ToDynamicArray();
castInSelect.Dump();

---

Output in LINQPad:

回答2:

Try this:

var arg0 = Expression.Parameter(typeof(Color), "x");
var expr = DynamicExpressionParser.ParseLambda(new[] { arg0 }, typeof(Color?), "x");
var qry2 = range.AsQueryable().Select("@0(it)", expr);

Also see https://github.com/StefH/System.Linq.Dynamic.Core/wiki/Dynamic-Expressions#dynamic-lambda-invocation

回答3:

I would like to suggest that you can use the following snippet to identify whether the input number is part of the enumeration.

public static bool IsValuePresent<T>(int number) where T : Enum
{
    return !Enum.GetValues(typeof(T)).Cast<int>().Contains(number);
}

The following might be the calls to check if the input value is present in the enumeration

EnumerationConverter.IsValuePresent<Color>(99); //returns False
EnumerationConverter.IsValuePresent<Color>(2); //returns True

Once a False is obtained in the above statements, we can set the value to be null or else, we can do a simple generic type converter from integer to the enumeration like the code given below.

public static T GetAsEnum<T>(int number) where T : Enum
{
    if (Enum.IsDefined(typeof(T), number))
    {
        return (T)Enum.ToObject(typeof(T), number);
    }
    return default(T);
}

Based on the understanding that you have to check for valid enumeration value and then do a conversion, the above solution can be tried, incase of any other aspect, please post here to update.

来源：https://stackoverflow.com/questions/60692533/convert-my-value-type-to-nullable-equivalent