Добавьте файлы проекта.

Yuna.Website/Yuna.Website.Server/Storage/Repositories/User/IUserRepository.cs

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+namespace Yuna.Website.Server.Storage.Repositories.User
+{
+    public interface IUserRepository
+    {
+        public Task<Model.User?> GetById(long id);
+        public Task<Model.User?> GetByUsername(string username);
+        public Task<IReadOnlyList<Model.User>> GetList();
+        public Task<Model.User?> Create(Model.User? user);
+        public Task<Model.User?> Update(Model.User? user);
+        public Task<Model.User?> Delete(long id);
+    }
+}

Yuna.Website/Yuna.Website.Server/Storage/Repositories/User/UserRepository.cs

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+
+using Dapper;
+using Newtonsoft.Json.Linq;
+using Yuna.Website.Server.Model;
+
+namespace Yuna.Website.Server.Storage.Repositories.User
+{
+    public class UserRepository : IUserRepository
+    {
+        private readonly DapperContext _context;
+
+        public UserRepository(DapperContext context)
+        {
+            _context = context;
+        }
+        public async Task<Model.User?> Create(Model.User? user)
+        {
+            var query =
+               $@"INSERT INTO ""Yuna_Users""
+                (""Username"", ""HashedPassword"", ""IsAdmin"")
+                VALUES
+                (@{nameof(Model.User.UserName)}, @{nameof(Model.User.HashedPassword)}, @{nameof(Model.User.IsAdmin)})
+                RETURNING *
+            ";
+
+            var result = await _context.Connection.QuerySingleAsync<Model.User?>(query, user);
+            return result;
+        }
+
+        public Task<Model.User?> Delete(long id)
+        {
+            throw new NotImplementedException();
+        }
+
+        public async Task<Model.User?> GetById(long id)
+        {
+            var query =
+                $@"SELECT 
+                    u.""Id"" as {nameof(Model.User.Id)}, 
+                    u.""Username"" as {nameof(Model.User.UserName)}, 
+                    u.""HashedPassword"" as {nameof(Model.User.HashedPassword)},
+                    u.""IsAdmin"" as {nameof(Model.User.IsAdmin)}
+                    FROM ""Yuna_Users"" u
+                    WHERE u.""Id"" = {id}
+                    LIMIT 1";
+
+            var result = await _context.Connection.QuerySingleAsync<Model.User>(query);
+            return result;
+        }
+
+        public async Task<Model.User?> GetByUsername(string username)
+        {
+            var query =
+               $@"SELECT 
+                    u.""Id"" as {nameof(Model.User.Id)}, 
+                    u.""Username"" as {nameof(Model.User.UserName)}, 
+                    u.""HashedPassword"" as {nameof(Model.User.HashedPassword)},
+                    u.""IsAdmin"" as {nameof(Model.User.IsAdmin)}
+                    FROM ""Yuna_Users"" u
+                    WHERE LOWER(u.""Username"") = '{username.ToLower()}'
+                    LIMIT 1";
+
+            var result = await _context.Connection.QuerySingleOrDefaultAsync<Model.User>(query);
+            return result;
+        }
+
+        public async Task<IReadOnlyList<Model.User>> GetList()
+        {
+            var query = 
+            $@"SELECT 
+                    u.""Id"" as {nameof(Model.User.Id)}, 
+                    u.""Username"" as {nameof(Model.User.UserName)}, 
+                    u.""HashedPassword"" as {nameof(Model.User.HashedPassword)},
+                    u.""IsAdmin"" as {nameof(Model.User.IsAdmin)}
+                    FROM ""Yuna_Users"" u";
+
+            var result = await _context.Connection.QueryAsync<Model.User>(query);
+            return result.ToList();
+        }
+
+        public async Task<Model.User?> Update(Model.User? user)
+        {
+            var query = $@"
+                        UPDATE ""Yuna_Users""
+                        SET 
+                        ""Username"" = @{nameof(Model.User.UserName)},
+                        ""HashedPassword"" = @{nameof(Model.User.HashedPassword)},
+                        ""IsAdmin"" = {nameof(Model.User.IsAdmin)}
+                        ""WHERE Id"" = @{nameof(Model.User.Id)}
+                        RETURNING *
+                        ";
+
+            var result = await _context.Connection.QuerySingleOrDefaultAsync<Model.User>(query, user);
+            return result;
+        }
+    }
+}