Use Lambda expressions to efficiently filter lists containing multiple keywords

You need to search multiple keywords in one database field. This can be achieved with a SQL query like this:

select * from MainList
where Comment like '%keyword1%'
or Comment like '%keyword2%'
... 以此類推，直到最后一個(gè)關(guān)鍵詞。

In C# you can do this:

var newList = MainList
   .FilterByItems(keywords, (m, k) => m.Comments.Contains(k), true)
   .ToList();

The following is the implementation of the FilterByItems extension method:

public static class QueryableExtensions
{
    public static IQueryable<T> FilterByItems<T, TItem>(this IQueryable<T> query, IEnumerable<TItem> items,
        Expression<Func<T, TItem, bool>> filterPattern, bool isOr)
    {
        Expression? predicate = null;
        foreach (var item in items)
        {
            var itemExpr = Expression.Constant(item);
            var itemCondition = ExpressionReplacer.Replace(filterPattern.Body, filterPattern.Parameters[1], itemExpr);
            if (predicate == null)
                predicate = itemCondition;
            else
            {
                predicate = Expression.MakeBinary(isOr ? ExpressionType.OrElse : ExpressionType.AndAlso, predicate,
                    itemCondition);
            }
        }

        predicate ??= Expression.Constant(false);
        var filterLambda = Expression.Lambda<Func<T, bool>>(predicate, filterPattern.Parameters[0]);

        return query.Where(filterLambda);
    }

    class ExpressionReplacer : ExpressionVisitor
    {
        readonly IDictionary<Expression, Expression> _replaceMap;

        public ExpressionReplacer(IDictionary<Expression, Expression> replaceMap)
        {
            _replaceMap = replaceMap ?? throw new ArgumentNullException(nameof(replaceMap));
        }

        [return: NotNullIfNotNull(nameof(node))]
        public override Expression? Visit(Expression? node)
        {
            if (node != null && _replaceMap.TryGetValue(node, out var replacement))
                return replacement;
            return base.Visit(node);
        }

        public static Expression Replace(Expression expr, Expression toReplace, Expression toExpr)
        {
            return new ExpressionReplacer(new Dictionary<Expression, Expression> { { toReplace, toExpr } }).Visit(expr);
        }

        public static Expression Replace(Expression expr, IDictionary<Expression, Expression> replaceMap)
        {
            return new ExpressionReplacer(replaceMap).Visit(expr);
        }

        public static Expression GetBody(LambdaExpression lambda, params Expression[] toReplace)
        {
            if (lambda.Parameters.Count != toReplace.Length)
                throw new InvalidOperationException();

            return new ExpressionReplacer(Enumerable.Range(0, lambda.Parameters.Count)
                .ToDictionary(i => (Expression)lambda.Parameters[i], i => toReplace[i])).Visit(lambda.Body);
        }
    }
}

The above is the detailed content of How Can I Efficiently Filter a List Using Multiple Keywords with a Lambda Expression in C#?. For more information, please follow other related articles on the PHP Chinese website!