LINQ to Entities-无法识别的方法

Question

I'm facing a problem with executing a query against a Queryable dataset. The original call looks like this:

books = books.Where(b => (GetPropertyValue(b, filter.CategoryProperties.DbName) == null ? 0 : Convert.ToInt32(GetPropertyValue(b, filter.CategoryProperties.DbName))) < Convert.ToInt32(filter.Value));

This gets me the Method not recognized error. This of course is expected due to the call to GetPropertyValue. I then read that I should build the expression tree myself. That result in the following code:

public IQueryable<Books> GetExpression(IQueryable<Books> books, BookCategoryMapping filter)
    {
        var booksExpression = Expression.Parameter(typeof(Books), "b");
        var methodInfo = this.GetType().GetMethod("GetPropertyValue");
        var value = Expression.Call(methodInfo, booksExpression, Expression.Constant(filter.CategoryProperties.DbName));
        var left = Expression.Constant(value);
        var right = Expression.Constant(filter.Value);
        var expression = Expression.Equal(left, right);
        var whereExpression = Expression.Call(typeof(Queryable), "Where", new Type[] { books.ElementType }, books.Expression, Expression.Lambda<Func<Books, bool>>(expression, new ParameterExpression[] { booksExpression }));
        return books.Provider.CreateQuery<Books>(whereExpression);
    }

Only problem being, that I get the same error. It seems like the following line only produces an expression and not the value of said expression.

var value = Expression.Call(methodInfo, booksExpression, Expression.Constant(filter.CategoryProperties.DbName));

Any help producing the correct expression tree would be greatly appreciated :-)

EDIT: Here's the GetPropertyValue method:

public static object GetPropertyValue(object obj, string name)
    {
        try
        {
            return obj.GetType().GetProperty(name)?.GetValue(obj, null);
        }
        catch (Exception ex)
        {
            LogManager.Log(LogLevel.Error, null, ex);
        }

        return obj;
    }

Answer 1

The method below generates an Expression<Func<Book, bool>> which determines whether a given property of a Book is less than a given (constant) value. The error-checking code you currently have in GetPropertyValue can probably be replaced by catching the ArgumentException which will be thrown when you try to create an expression for a nonexistent property.

Note that I have assumed that the property you're accessing is genuinely numeric, and that your call to Convert.ToInt32 was only necessary because your GetPropertyValue method returns an object .

Expression<Func<Book, bool>> GenerateLessThanExpression(string propertyName, int value)
{
    var parameter = Expression.Parameter(typeof (Book));
    var property = Expression.Property(parameter, propertyName);
    var comparison = Expression.LessThan(property, Expression.Constant(value));

    return Expression.Lambda<Func<Book, bool>>(comparison, parameter);
}

Here's a sample usage to return only very short books:

var filter = GenerateLessThanExpression("Pages", 5);
var filtered = books.Where(filter);