How to use MethodInfo and lifttonull in Expression equality on c#

问题

I'm trying to use Expression.NotEqual() (here)

But I can't understand how to use the methodInfo.

I'm trying to make a comparition to a possible bool?.

This is the property that I want to evaluate, wich was added to database after release (creating null's as well as true and falses:

public bool? Deactivated { get; set; } = false;

I'm using a working expression builder with generic that accepts parameters sent in a object that has a List of WhereParameters like bellow:

List<WhereParameters> whereList = new List<WhereParameters>();

        whereList.Add(new WhereParameters
        {
            field = "ID",
            field_Value_Relation = "notEqual",
            value = Guid.Empty
        });
        whereList.Add(new WhereParameters
        {
            field = "Deactivated",
            field_Value_Relation = "notEqualorNull", //orNull => nullable
            value = true
        });

These parameters are used in the builder:

 public List<M> getExpression(QueryParameters<M> QueryParams)
    {

        

        //GET TYPE FROM GENERIC M
        Type modelType = QueryParams.modelType;


        //QUERY DB with GENERIC TYPE
        IQueryable<M> queryableData = (IQueryable<M>)QueryParams.db.Set(modelType);

        // Compose the expression tree that represents the parameter to the predicate.  

        //CREATE EXPRESSION PARAMETER
        ParameterExpression pe = Expression.Parameter(typeof(M), "param"); 

        // Create an expression tree that represents the expression 
        //if (QueryParams.whereParameters != null && QueryParams.whereParameters.Count > 0)
        //{

            int w = 0;
        foreach (WhereParameters whereParam in QueryParams.whereParameters) { 
        //BLOCK A.1 WHERE
        //Create Property on the Left of equality
        Expression left1 = Expression.Property(pe, whereParam.field.ToLower());
        //Create Property on the right of equality
        Expression right1 = Expression.Constant(whereParam.value);

            //Expresses the relation between left and right

            //Expression e1;
                switch (whereParam.field_Value_Relation)
                {
                    case "notEqualOrNull":
                        predicateBody = Expression.NotEqual(left1, right1, true , ????????);
                        break;
                }
}

It is throwing a System.InvalidOperationException: The binary operator NotEqual is not defined for the types 'System.Nullable`1[System.Boolean]' and 'System.Boolean'.

I don't know how to make this work with nullables. I understood the islifted param, but can't manage to understand what to send in the last parameter of Expression.NotEqual, as MethodInfo. Can someone please show me examples on how this can be done?

Thank you all

回答1:

Encountered an answer here

My debug code before refactoring (maintain everything, just changing the Switch, case):

                 case "notEqualOrNull":
             
                       //this block will be promoted out of here
                        var field = whereParam.field;
                        var fieldValue = whereParam.value;
                        var prop = modelType.GetProperty(field);
                        var isNullable = prop.PropertyType.IsNullableType();
                       
                        var member = Expression.Property(pe, field);


                        var filter1 =
                        Expression.Constant(Convert.ChangeType(fieldValue, member.Type.GetGenericArguments()[0]));
                        Expression typeFilter = Expression.Convert(filter1, member.Type);
                        
                        //this is a proxy of the code that will be here after refactoring
                        andPredicateBody = Expression.NotEqual(left1, typeFilter);
                        predicateBody = Expression.OrElse(predicateBody, andPredicateBody);

                        break;

This will be refactored, in order to be more general, once it can test all nullables. the only change in the 'switch case' will be the var right1 or typefilter.

来源：https://stackoverflow.com/questions/63524188/how-to-use-methodinfo-and-lifttonull-in-expression-equality-on-c-sharp

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