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ASP.NET Search Form - Dynamic Linq to SQL?

I have a search form that allows users to search across several different fields in several ways. Here is an example of my code.

var claims = from c in db.Claims select c;

switch (ddlSearchField.Text)
{
    case "StartsWith":
        claims = claims.Where(c => c.companyFileID.StartsWith(txtSearchBox.Text));
        break;

    case "Equals":
        claims = claims.Where(c => c.companyFileID == txtSearchBox.Text);
        break;

    case "Contains":
        claims = claims.Where(c => c.companyFileID.Contains(txtSearchBox.Text));
        break;
}

I have about ten different fields that the user can perform searches, so my external switch statement is pretty big. There must be a more elegant way to achieve this.

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3 answers

You can reorganize some parts of the code by creating an extension method. Something like that:

static class QueryableExtensions
{
    private static MethodInfo StringContainsMethod;
    private static MethodInfo StringStartsWithMethod;

    static QueryableExtensions()
    {
        Type[] singleStringParam = new[] {typeof(string)};
        StringContainsMethod = typeof(string).GetMethod("Contains", singleStringParam);
        StringStartsWithMethod = typeof(string).GetMethod("StartsWith", singleStringParam);
    }

    public static IQueryable<T> AppendTextFilter<T>(this IQueryable<T> queryable, Expression<Func<T, string>> memberSelector, string condition, string value)
    {
        Expression expression = null;
        switch (condition)
        {
            case "StartsWith":
                expression = Expression.Call(
                                memberSelector.Body,
                                StringStartsWithMethod,
                                Expression.Constant(value));
                break;

            case "Equals":
                expression = Expression.Equal(
                                memberSelector.Body,
                                Expression.Constant(value));
                break;

            case "Contains":
                expression = Expression.Call(
                                memberSelector.Body,
                                StringContainsMethod,
                                Expression.Constant(value));
                break;

            default:
                throw new NotSupportedException(string.Format("'{0}' is not a supported condition", condition));
        }

        var lambda = Expression.Lambda<Func<T, bool>>(
                        expression,
                        memberSelector.Parameters);
        return queryable.Where(lambda);
    }
}

Then you can use it as follows:

var claims = db.Claims
             .AppendTextFilter(c => c.companyFileID, ddlSearchField.Text, txtSearchBox.Text)
             .AppendTextFilter(c => c.someOtherProperty, ddlOtherSearchField.Text, txtOtherSearchBox.Text)
             ...;
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You can make a dictionary of your possible predicates:

Dictionary<string, Func<string, Expression<Func<Claim, bool>>>> map = new Dictionary<string, Func<string, Expression<Func<Claim, bool>>>>() {
    { "StartsWith", t => c => c.companyFileID.StartsWith(t) },
    { "Equals",     t => c => c.companyFileID == t },
    { "Contains",   t => c => c.companyFileID.Contains(t) }
};

What can be used as follows:

var search = ddlSearchField.Text;
var text = txtSearchBox.Text;

var claims = from c in db.Claims select c;

Func<string, Expression<Func<Claim, bool>>> predicate = null;
if(dict.TryGetValue(search, out predicate))
    claims = claims.Where(predicate(text));
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