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I had the following function to be mocked.

public interface IRepository

 Task<IEnumerable<Item>> GetItems(int total);

And my mocking code was

private readonly IEnumerable<Item> stubList = new List<Item> new Item ;

mockRepository = Mock.Of<IRepository>(r => r.GetItems(50) == Task.FromResult(stubList));

It worked on both my desktop (Visual studio 2017) and msbuild (MSBuild auto-detection: using msbuild version '15.8.169.51996' from 'C:Program Files (x86)Microsoft Visual Studio2017BuildToolsMSBuild15.0bin') on Jenkin server.

Now the method has been changed to

public interface IRepository

 Task<(IEnumerable<Item>, int)> GetItems(int total);

And the mocking code was changed to

private readonly IEnumerable<Item> stubList = new List<Item> new Item ;

var m = (stubList, 1);
mockRepository = Mock.Of<IRepository>(r => r.GetItems(50) == Task.FromResult(m));

It still works on my desktop (visual studio 2017). But msbuild failed with the following error message?

error CS0019: Operator '==' cannot be applied to operands of type
'Task<(IEnumerable<Item>, int)>' and 'Task<IEnumerable<Item>>'

build.log:

CoreResGen:
"C:Program FilesMicrosoft SDKsWindowsv6.0AbinResgen.exe" /useSourcePath /r:"D:Jenkinsworkspace...packagesDocumentFormat.OpenXml.2.8.1libnet35DocumentFormat.OpenXml.dll" /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727Microsoft.JScript.dll /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727mscorlib.dll /r:"C:Program Files (x86)Reference AssembliesMicrosoftFrameworkv3.5System.Core.dll" /r:"C:Program Files (x86)Reference AssembliesMicrosoftFrameworkv3.5System.Data.DataSetExtensions.dll" /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.Data.dll /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.Design.dll /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.dll /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.Drawing.dll /r:"C:Program Files (x86)Reference AssembliesMicrosoftFrameworkv3.0System.Runtime.Serialization.dll" /r:"C:Program Files (x86)Reference AssembliesMicrosoftFrameworkv3.0System.ServiceModel.dll" /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.Web.Services.dll /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.Windows.Forms.dll /r:C:WindowsMicrosoft.NETFrameworkv2.0.50727System.Xml.dll /r:"C:Program Files (x86)Reference AssembliesMicrosoftFrameworkv3.5System.Xml.Linq.dll" /r:"C:Program Files (x86)Reference AssembliesMicrosoftFrameworkv3.0WindowsBase.dll" /compile ComponentsCheckedComboBoxPopupComboBox.resx,objReleasePresentationControls.PopupComboBox.resources ComponentsDGVDgvDesignerColumnList.resx,objReleaseInfrastructure.DgvDesignerColumnList.resources ComponentsDGVfrmGridColumnsExt.resx,objReleaseInfrastructure.frmGridColumnsExt.resources ComponentsDGVdgv.resx,objReleaseInfrastructure.DGV.resources ComponentsDGVfrmChangeGridState.resx,objReleaseInfrastructure.frmChangeGridState.resources ComponentsDGVfrmGridColumns.resx,objReleaseInfrastructure.frmGridColumns.resources ComponentsUserControl_Folder.resx,objReleaseInfrastructure.UserControl_Folder.resources frmDropDownBox.resx,objReleaseInfrastructure.frmDropDownBox.resources frmUsersChangeHistory.resx,objReleaseInfrastructure.frmUsersChangeHistory.resources frmUserPermissions.resx,objReleaseInfrastructure.frmUserPermissions.resources frmUserRegProdGroups.resx,objReleaseInfrastructure.frmUserRegProdGroups.resources frmErrorBox.resx,objReleaseInfrastructure.frmErrorBox.resources frmGridBox.resx,objReleaseInfrastructure.frmGridBox.resources frmInputBox.resx,objReleaseInfrastructure.frmInputBox.resources frmLongTask.resx,objReleaseInfrastructure.frmLongTask.resources frmNoteBox.resx,objReleaseInfrastructure.frmNoteBox.resources ComponentsMonthPicker.resx,objReleaseInfrastructure.MonthPicker.resources frmUserGroups.resx,objReleaseInfrastructure.frmUserGroups.resources frmUsers.resx,objReleaseInfrastructure.frmUsers.resources PropertiesResources.resx,objReleaseInfrastructure.Properties.Resources.resources ReportsfrmEditReports.resx,objReleaseInfrastructure.frmEditReports.resources ReportsfrmJobsMaintenance.resx,objReleaseInfrastructure.frmJobsMaintenance.resources ReportsfrmRunReports.resx,objReleaseInfrastructure.frmRunReports.resources ReportsfrmSelectReport.resx,objReleaseInfrastructure.frmSelectReport.resources ReportsfrmShowReportLog.resx,objReleaseInfrastructure.frmShowReportLog.resources

Try using the more verbose approach instead of the LINQ to Mocks

private readonly IEnumerable<Item> stubList = new List<Item> new Item ;

//...

var expected = (stubList, 1);

var mock = new Mock<IRepository>();
mock
 .Setup(_ => _.GetItems(50))
 .ReturnsAsync(expected); 

IRepository mockRepository = mock.Object;

//...

The framework may be having issues trying to evaluate the expression with the new syntax

When I try your code:

class Program

 static void Main(string args)
 
 var stubList = new List<Item>
 
 new Item()
 ;

 var m = (stubList, 1);

 var mockRepository = Mock.Of<IRepository>(r => r.GetItems(50) == Task.FromResult(m));
 


interface IRepository

 Task<(IEnumerable<Item>, int)> GetItems(int total);


class Item

I'm getting different compiler error:

CS0019 Operator '==' cannot be applied to operands of type 'Task<(IEnumerable, int)>' and 'Task<(List stubList, int)>'

This happens because you compare Task<(List<Item>, int)> and Task<(IEnumerable<Item>, int)>. Task<T> is not co-variant (opposite to List<T> for instance) so the error is expected.

But why your question contains different compiler error?

If I change the code to this:

var m = ((IEnumerable<Item>)stubList, 1);

or this:

(IEnumerable<Item>, int) m = (stubList, 1);

then it complies fine.