我可以在LINQ Where子句中使用属性吗？

Question

First off, I apologize if the title of this is trash. I'm new to C# and ASP and I'm not even sure how to ask this question.

I'm trying to incorporate a search feature to my web page. I have a text box that will take a search term that the user wants. A drop down will select which FIELD of my database table it will search in. So if there are last names and first names, the user can choose which to search in. I have a property called SearchField{get; set;} that will hold the value the user selects in the drop down but when I try to use this in my LINQ statement along with the Contains(blah) I get an error.

I've tried very little since I really don't know much about C# or what the error is even telling me.

cshtml file

<form>
    <p>
        <select asp-for="SearchField" class="form-control">
            <option value="FirstName">First Name</option>
            <option value="LastName">Last Name</option>
            <option value="Salary">Salary</option>
            <option value="Gender">Gender</option>
            <option value="Department">Department</option>
            <option value="Location">Location</option>
            <option value="Performance">Performance</option>
        </select>
        Search: <input type="text" asp-for="SearchString" class="form-control" />
        <input type="submit" value="Filter" class="form-control" />
    </p>
</form>

cs file

   [BindProperty(SupportsGet = true)]
   public string SearchField { get; set; }

   public async Task OnGetAsync()
    {
        var employees = from x in _context.Employee
                     select x;
        if (!string.IsNullOrEmpty(SearchString))
        {
            employees = employees.Where(x => x.SearchField.Contains(SearchString));
        }
        Employee = await employees.ToListAsync();
    }

Error: Object does not contain a definition for 'Contains' and the best known extension method overload Queryable.Contains..... requires a receiver of type IQueryable

Answer 1

You can use reflection in C# to get the property based on your SearchField and compare the value of the property with your SearchString. https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/reflection

In your case:

public async Task OnGetAsync()
    {
        var employees = from x in _context.Employee
                     select x;
        if (!string.IsNullOrEmpty(SearchString))
        {
            employees = employees.Where(x => x.GetType().GetProperty(SearchField).GetValue(x, null).ToString() == SearchString);
        }
        Employee = await employees.ToListAsync();
    }

You could also use String.Contains in the following way:

employees = employees.Where(x => x.GetType().GetProperty(SearchField).GetValue(x, null).ToString().Contains(SearchString, StringComparison.InvariantCultureIgnoreCase));

Answer 2

You need to dynamically create Expression tree which will be filtering your table and return only matched values.

Here is what you need for string properties (it's deliberately spread over many lines):

// Expression to construct: (Employee parameter) => parameter.GetPropertyValue(searchField).Contains(SearchValue)
var parameter = Expression.Parameter(typeof(Employee));
var employeePropertyValue = Expression.Property(parameter, searchField);

var constainsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
var contains = Expression.Call(employeePropertyValue, containsMethod, Expression.Constant(searchString));
var whereCondition = (Expression<Func<Employee, bool>>)Expression.Lambda(contains, parameter);

// filtering
employees = employees.Where(whereCondition);

For non-string properties, you will need to construct different condition depending of property type (for example Enum property will be stored in database in integer so you need to convert SearchValue to Gender and use Expression.Equal)