CODEWITHSUNDEEP
c#expression-trees
tia
tia

Reputation: 9698

Expression tree condition conversion with selector

I have an entity class like

public class BookPage {
    public int PageIndex { get; set; }
}

then I have an expression:

Expression<Func<int, bool>> pageIndexCondition = idx => idx == 1;

and the expression I want:

Expression<Func<BookPage, bool>> pageCondition = bookPage => bookPage.PageIndex == 1;

The question: How do I use pageIndexCondition to do LINQ-to-SQL query, or how can I convert pageIndexCondition into pageCondition?

Edit: Another solution that would be less elegant, but still meet my requirement is:

Expression<Func<T, bool>> GetPageIndexCondition(Expression<Func<T, int>> selector) {
     return (T item) => selector(item) < 10; // This won't work because selector is Expression, so how to implement this correctly?
}

...

var pageCondition = GetPageIndexCondition(page => page.PageIndex);

Upvotes: 2

Views: 180

Answers (2)

Shlomo
Shlomo

Reputation: 14350

I like doing these things, though as others have said, there's probably more efficient and better ways to do it: 

void Main()
{
    Expression<Func<int, bool>> pageIndexCondition = idx => idx == 1;
    Expression<Func<BookPage, bool>> converted = ExpressionConverter.Convert(pageIndexCondition);
}

public class ExpressionConverter : ExpressionVisitor
{
    public static Expression<Func<BookPage, bool>> Convert(Expression<Func<int, bool>> e)
    {
        var oldParameter = e.Parameters.First();
        var newParameter = Expression.Parameter(typeof(BookPage), "bp");
        Expression<Func<BookPage, int>> x = (BookPage bp) => bp.PageIndex;
        var property = ((x.Body as MemberExpression).Member as PropertyInfo);
        var memberAccess = Expression.Property(newParameter, property);

        var converter = new ExpressionConverter(oldParameter, memberAccess);
        return (Expression<Func<BookPage, bool>>)Expression.Lambda(converter.Visit(e.Body), newParameter);
    }

    private ParameterExpression pe;
    private Expression replacement;

    public ExpressionConverter(ParameterExpression pe, Expression replacement)
    {
        this.pe = pe;
        this.replacement = replacement;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if(node == pe)
            return replacement;

        return base.VisitParameter(node);
    }

}

Upvotes: 1

Sam Leach
Sam Leach

Reputation: 12956

var pages = new List<BookPage>
                {
                    new BookPage { PageIndex = 1 }, 
                    new BookPage { PageIndex = 2 }
                };

Expression<Func<BookPage, bool>> pageCondition = bookPage => bookPage.PageIndex == 1;

BookPage result = pages.AsQueryable().Single(pageCondition);

If you want a generic select by id you will have to do something like;

public virtual IEnumerable<TEntity> Get(Expression<Func<TEntity, bool>> filter = null)
{
    if (filter != null)
    {
        query = query.Where(filter);
    }
}

This goes in your generic repository.

Upvotes: 0

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