Filtering generic select with changeable parameters

Question

Filtering generic select with changeable parameters

c# entity-framework-core generics where-clause

Question

I'll keep it simple. I have an API endpoint that receives 0 to 3 parameters. The get method look like this:

[HttpGet("filtrar")]
public ActionResult<List<TipoDocumento>> GetAllPorFiltro(string sigla, int? status, string descricao)
{
    return _tipoDocumentoRepositorio.GetAllByFilter(????);
}

The _tipoDocumentoRepositorio is an interface that has DI, and the method GetAllByFilter() in the class that implements it looks like this:

public List<T> GetAllByFilter(Expression<Func<T, bool>> filter)
{
    return _someContexto.Set<T>()
        .Where(filter)
        .ToList();
}

The fact is: even though I put a function on ???? (like

f => f.Sigla.Equals(sigla)

), it keeps me returning an empty List. What am I doing wrong? Or what else should I do to make it work?

Obs: I can't put all of the code here cause it's not mine, but I can argue. To be sure: I'm using EF (and Migration and sort, I'm new in C#). And any links to other SO question that may answer my question is welcomed as well. Thanks in advance.

Ricardo Sampaio

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koryakinp · Accepted Answer · 9/12/2018 5:23:14 PM

Change your GetAllByFilter to just GetAll() and return just IQueriable<T>

Create an extension method:

public static IQueryable<T> WhereIf<T>(
   this IQueryable<T> source, bool condition, 
   Expression<Func<T, bool>> predicate)
{
    if (condition)
        return source.Where(predicate);
    else
        return source;
}

Use it like that:

[HttpGet("filtrar")]
public ActionResult<List<TipoDocumento>> GetAllPorFiltro(string sigla, int? status, string descricao)
{
     var res = _tipoDocumentoRepositorio
        .GetAll()
        .WhereIf(!String.IsNullOrEmpty(sigla), q => q.sigla == sigla)
        .WhereIf(status.HasValue, q => q.sigla == sigla.Value)
        .WhereIf(!String.IsNullOrEmpty(descricao), q => q.descricao == descricao)
        .ToList();
     ...
}

Filtering generic select with changeable parameters

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