add ToList

Tsi1.Api/Tsi1.BusinessLayer/Services/PostMessageService.cs

@@ -72,7 +72,7 @@ namespace Tsi1.BusinessLayer.Services
 
             var postMessageDtos = _mapper.Map<List<PostMessagePreviewDto>>(postMessages);
 
-            var userIds = postMessageDtos.Select(x => x.UserId);
+            var userIds = postMessageDtos.Select(x => x.UserId).ToList();
 
             var users = await _context.Users.Where(x => userIds.Contains(x.Id)).ToListAsync();
 

Tsi1.Api/Tsi1.BusinessLayer/Services/PostService.cs

@@ -72,7 +72,7 @@ namespace Tsi1.BusinessLayer.Services
 
             var postDtos = _mapper.Map<List<PostPreviewDto>>(posts);
 
-            var userIds = postDtos.Select(x => x.UserId);
+            var userIds = postDtos.Select(x => x.UserId).ToList();
 
             var users = await _context.Users.Where(x => userIds.Contains(x.Id)).ToListAsync();