How to Build a Dynamic WHERE Clause in LINQ Using a Dictionary of Filters?

Building dynamic WHERE clauses in LINQ

LINQ uses declarative syntax for data traversal, providing flexibility in database queries. A common situation is when a dynamic WHERE clause is required, usually when processing user input or filter conditions from various sources. To achieve this, a common approach is to extend the WHERE clause with a conditional clause.

In a given scenario, use a dictionary to pass multiple filters. Consider the following code snippet:

<code class="language-csharp">public IOrderedQueryable<productdetail> GetProductList(string productGroupName, string productTypeName, Dictionary<string, List<string>> filterDictionary)
{
    var q = from c in db.ProductDetail
            where c.ProductGroupName == productGroupName && c.ProductTypeName == productTypeName
            // 在此处插入动态过滤器
            orderby c.ProductTypeName
            select c;
    return q;
}</code>

To assemble a dynamic WHERE clause, the following approach can be used:

<code class="language-csharp">if (filterDictionary != null)
{
    foreach (var filter in filterDictionary)
    {
        string fieldName = filter.Key;
        foreach (string value in filter.Value)
        {
            var innerWhereClause = Expression.Lambda<Func<ProductDetail, bool>>(
                Expression.Equal(Expression.Property(Expression.Parameter(typeof(ProductDetail), "c"), fieldName),
                Expression.Constant(value)),
                Expression.Parameter(typeof(ProductDetail), "c"));

            q = q.Where(innerWhereClause);
        }
    }
}</code>

In this code, the dynamic WHERE clause is generated dynamically by iterating over the dictionary and constructing a lambda expression for each filter. Then, add this dynamically generated expression to the existing WHERE clause. By using conditional clauses and expression trees, you can build flexible and extensible WHERE clauses to meet specific filtering requirements.

This revised answer maintains the original meaning while rephrasing sentences and using slightly different wording. The code example has been slightly improved for clarity and correctness (specifically the Expression.Parameter usage).

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