How can I create a public static method while implementing an interface in c#?

Question

I have the following interface

public interface IQueryBuilder
{
    SqlCommand Build(IReportDataSource dataSource, List<IReportColumn> columns, List<IReportRelationMapping> relationMappings, IReportFilterMapping filters, List<IReportColumn> columsToSortBy, ReportFormat reportFormat);

    string GetSqlClause(List<IReportFilter> reportFilters, ref List<IDataParameter> sqlParams);
}

However, I would like to be able to access the method GetSqlClause in the implementation directly.

Here is how I implemented the above

public class QueryBuilder : IQueryBuilder
{
    public SqlCommand Build(IReportDataSource dataSource, List<IReportColumn> columns, List<IReportRelationMapping> relationMappings, IReportFilterMapping filters, List<IReportColumn> columsToSortBy, ReportFormat reportType)
    {
        //Do something awsome!!!
        string sqlQuery = "";

        List<IDataParameter> sqlParameters = new List<IDataParameter>();

        return this.GetSqlCommand(sqlQuery, sqlParameters);
    }

    private SqlCommand GetSqlCommand(string sqlQuery, List<IDataParameter> sqlParams)
    {
        var command = new SqlCommand(sqlQuery);

        foreach (IDataParameter dataParameter in sqlParams)
        {
            command.Parameters.Add(dataParameter);
        }

        return command;
    }


    public static string GetSqlClause(List<IReportFilter> reportFilters, ref List<IDataParameter> sqlParams)
    {
        string sqlFilter = "";

        if (reportFilters != null && reportFilters.Any())
        {
            //At this point we know there are some filter to add to the list

            var firstFilter = reportFilters.First();

            foreach (var reportFilter in reportFilters)
            {

                var parameter = GenerateDbParameter("p" + sqlParams.Count, reportFilter.FormattedValue, SqlDbType.NVarChar);
                ....
                ....
            }
        }

        return sqlFilter;
    }

    private static IDataParameter GenerateDbParameter(string parameterName, object parameterValue, SqlDbType parameterType)
    {
        if (string.IsNullOrEmpty(parameterName) || parameterValue == null)
        {
            throw new ArgumentException();
        }
        var parameter = new SqlParameter("@" + parameterName, parameterType)
        {
            Value = parameterValue
        };

        return parameter;
    }


}

Because I am using static on my GetSqlClause method I get an error

cannot implement an interface member because it is static.

What is a good work around to this problem? How can I access my GetSqlClause directly?

Is there a reason you want it to be static? Does it need to be? Just don't make it static. — Maurice Reeves, Jul 18 '16 at 19:05
having `static` members in an `interface` doesn't make sense. Why can't you use `QueryBuilder.GetSqlClause` instead of `IQueryBuilder.GetSqlClause` — Sam I am says Reinstate Monica, Jul 18 '16 at 19:06
Side note: Passing `System.List` as `ref` parameter is a poor design. Pass it regularly, or even better, as `IList`. — Mr Anderson, Jul 18 '16 at 19:12
@Jaylen, I used to wonder this to. My problem was that I didn't understand the point of interfaces. You should read up on Test-Driven Design and Dependency Injection. — user2023861, Jul 18 '16 at 19:25

Lucas Trzesniewski · Answer 1 · 2016-07-18T19:22:21.667

If you want your method to be static and implement an interface, the language won't let you do that, but you can work around it in several ways:

If accessing the whole implementation from a static context makes sense, you can simply declare a singleton object:
```
public static QueryBuilder Instance { get; } = new QueryBuilder();
```
And now you have access to the QueryBuilder.Instance static property, whose value implements IQueryBuilder.

Through an explicit method implementation. This is an additional instance method which implements the interface method, and whose signature won't conflict with another method in the class.

string IQueryBuilder.GetSqlClause(List<IReportFilter> reportFilters, ref List<IDataParameter> sqlParams)
{
    // Call the static method, this is *not* a recursive call
    return GetSqlClause(reportFilters, ref sqlParams);
}

The general syntax of an explicit implementation is:

ReturnType InterfaceName.MethodName(ParamType param)
{
    ...
}

Another (simpler) solution would be to use two distinct method names.

@Servy yes, the instance method calls the static one. This answers the question the way I understood it. — Lucas Trzesniewski, Jul 18 '16 at 19:08
@Servy I rewrote the answer to hopefully make my point more clear, it it better now? Even though the design may be questionable, it could potentially prove useful in some circumstances. — Lucas Trzesniewski, Jul 18 '16 at 19:26

score -1 · Answer 2 · answered Jul 18 '16 at 19:07

-1

C# doesn't support static members implementing interface methods.

answered Jul 18 '16 at 19:07

Daniel A. Thompson

1,904
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That is correct, but StackOverflow is not a trivia game. Static members in an interface don't even make sense, but from your answer, the OP might assume that it's merely not supported – Sam I am says Reinstate Monica Jul 18 '16 at 19:08

How can I create a public static method while implementing an interface in c#?

2 Answers2