SSRS Report Viewer doesn't detect datasource?
SELECT TOP 10 City, COUNT(City) FROM [MYDB].[dbo].[ResidentialAddress] WHERE StateName = 'WA' OR StateName = 'Washington' GROUP BY City ORDER BY COUNT(City) DESC

this.MyReportViewer.Reset();
            this.MyReportViewer.ProcessingMode = ProcessingMode.Local;
            this.MyReportViewer.AsyncRendering = false;
            this.MyReportViewer.LocalReport.LoadReportDefinition(new StreamReader("H:\DataReportsViewer\DataReportsViewer\" + RDLFileList.SelectedValue));
            this.MyReportViewer.LocalReport.ReportPath = RDLFileList.SelectedValue;
            this.MyReportViewer.ShowReportBody = true;
            this.MyReportViewer.LocalReport.Refresh();

<DataSources>
    <DataSource Name="DataSource1">
      <ConnectionProperties>
        <DataProvider>SQL</DataProvider>
        <ConnectString>Data Source=MYSERVER;Initial Catalog=Ad_Dev</ConnectString>
        <IntegratedSecurity>true</IntegratedSecurity>
      </ConnectionProperties>
      <rd:SecurityType>Integrated</rd:SecurityType>
      <rd:DataSourceID>350f6976-9402-43fd-b8f8-8f809f116f84</rd:DataSourceID>
    </DataSource>
  </DataSources>
  <DataSets>
    <DataSet Name="DataSet1">
      <Query>
        <DataSourceName>DataSource1</DataSourceName>
        <CommandText>SELECT TOP 10 ResidentialAddress.City,COUNT(ResidentialAddress.City)
FROM ResidentialAddress
WHERE StateName = 'WA'
GROUP BY ResidentialAddress.City
ORDER BY COUNT(ResidentialAddress.City)</CommandText>
        <rd:UseGenericDesigner>true</rd:UseGenericDesigner>
      </Query>
      <Fields>
        <Field Name="City">
          <DataField>City</DataField>
          <rd:TypeName>System.String</rd:TypeName>
        </Field>
        <Field Name="ID">
          <DataField />
          <rd:TypeName>System.Int32</rd:TypeName>
        </Field>
      </Fields>
    </DataSet>
  </DataSets>

//Added line: TestDataSource runs same query and returns correctly loaded DataTable
            ReportDataSource reportDataSource = new ReportDataSource("DataSet1", TestDataSource());
            this.MyReportViewer.LocalReport.DataSources.Add(reportDataSource);