ParameterValue[] paramValues = new[] {new ParameterValue {Name = "RegionID", Value = "78" }}

_rs.SetExecutionParameters(paramValues , "en-us");
result = _rs.Render(format, null, out extension, out encoding, out mimeType, out warnings, out streamIDs);

public class Reports : IReports
{
    ReportExecutionService _rs;

    public Reports()
    {
         _rs = new ReportExecutionService();
        _rs.Credentials = System.Net.CredentialCache.DefaultCredentials;   

        param = GetReportParameters(reportName);     
    }

    private ParamValue[] GetReportParameters(string nameReport)
    {
        var report = GetReportInfo(nameReport);
        List<ParamValue> pv = new List<ParamValue>();
        if (report == null) return null;

        var param = report.Parameters;
        if (param != null)
        {
            foreach (ReportParameter p in param)
            {
                pv.Add(new ParamValue()
                {
                    Name = p.Name,
                    Label = p.Prompt,
                    Type = p.Type.ToString(),
                    Nullable = p.Nullable,
                    AllowBlank = p.AllowBlank,
                    MultiValue = p.MultiValue,
                    Required = (p.DefaultValues == null),
                    Values = GetValidValues(p),
                    Dependencies = p.Dependencies
                });
                SetDefaultValues(p.Name, GetValidValues(p), p.MultiValue);
            }
        }
        return pv.ToArray();
    }

    private void SetDefaultValues(string name, ParamValidValues[] values, bool multi)
    {
        if (values.Length > 0)
        {
            ParameterValue[] parameter = new ParameterValue[values.Length];
            for (int i = 0; i < values.Length; i++)
            {                    
                parameter[i] = new ParameterValue();
                parameter[i].Name = name;
                parameter[i].Value = values[i].Value;                    
            }
            _rs.SetExecutionParameters(parameter, "en-us");
        }
    }

}

public class Report
{
    public string Name { get; set; }
    public string Description { get; set; }
    public bool Rights { get; set; }
    public ParamValue[] Parameters { get; set; }
}
public class ParamValue
{
    public string Name { get; set; }
    public string Label { get; set; }
    public string Type { get; set; }
    public ParamValidValues[] Values { get; set; }
    public bool Nullable { get; set; }
    public bool MultiValue { get; set; }
    public bool AllowBlank { get; set; }
    public bool Required { get; set; }
    public bool Hide { get; set; }
    public string[] Dependencies { get; set; }
}
public class ParamValidValues
{
    public string Label { get; set; }
    public string Value { get; set; }
}

        private ExecutionInfo GetReportInfo(string nameReport)
        {
            try
            {
                var r = _rs.LoadReport(_reportPath + nameReport, null);
                return r;
            }
            catch { return null; }
        }

        private ParamValidValues[] GetValidValues(ReportParameter param)
        {
            List<ParamValidValues> values = new List<ParamValidValues>();
            ValidValue[] vals = param.ValidValues;
            if (vals != null)
            {
                foreach (ValidValue v in vals)
                    values.Add(new ParamValidValues() { Label = v.Label, Value = v.Value });
            }
            return values.ToArray();
        }