| FileNotFoundException assembly |
Could not load file or assembly 'X, Version=…' or one of its dependencies |
The runtime couldn’t find or load a referenced DLL — missing from the output, wrong version, or wrong platfor… |
| Unable to cast object of type |
System.InvalidCastException: Unable to cast object of type 'X' to type 'Y' |
An explicit cast failed because the object’s runtime type isn’t the target type. |
| NullReferenceException |
System.NullReferenceException: Object reference not set to an instance of an object |
Code dereferenced a variable that is `null` — the .NET equivalent of Java’s NullPointerException. |
| Cannot access a disposed context instance |
ObjectDisposedException: Cannot access a disposed context instance |
Code used a DbContext after its scope ended — usually async void, fire-and-forget tasks, or capturing a scope… |
| Index was out of range |
ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection |
Code accessed a List/collection position that doesn’t exist (IndexOutOfRangeException for arrays). |
| InvalidOperationException |
InvalidOperationException: Unable to resolve service for type 'X' while attempting to activate 'Y' |
ASP.NET Core’s dependency injection container can’t build Y because one of its constructor dependencies (X) w… |
| Access to the path is denied |
System.UnauthorizedAccessException: Access to the path 'x' is denied |
The process’s identity can’t read/write that file or folder — IIS app pool identity, container user, read-onl… |
| Input string was not in a correct format |
System.FormatException: Input string was not in a correct format |
int.Parse/decimal.Parse/Convert.ToInt32 got a string that isn’t a valid number — empty, contains text, or use… |
| NU1101 |
error NU1101: Unable to find package X. No packages exist with this id in source(s): nuget.org |
NuGet restore can’t find the package in any configured source — typo, private feed not configured/authenticat… |
| HttpClient.Timeout |
TaskCanceledException: The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing |
The HTTP call took longer than HttpClient.Timeout (100 s default), so .NET cancelled it. |
| 500.30 |
HTTP Error 500.30 - ASP.NET Core app failed to start |
IIS/Azure’s ASP.NET Core Module launched your app but it crashed during startup. |
| An unhandled error has occurred |
Blazor: An unhandled error has occurred. Reload 🗙 |
A Blazor component threw an exception; Blazor Server tears down the circuit (or WebAssembly logs to the conso… |
| NU1605 |
error NU1605: Detected package downgrade: X from 8.0.1 to 8.0.0. Reference the package directly from the project to select a different version |
A direct reference requests a lower version than a dependency needs, and NuGet treats the downgrade as an err… |
| DbUpdateException |
DbUpdateException: An error occurred while saving the entity changes. See the inner exception for details. |
SaveChanges failed in the database; the inner exception (SqlException/PostgresException) has the real reason… |
| requires a primary key to be defined |
EF Core: The entity type 'X' requires a primary key to be defined |
EF Core found an entity class without a key property it can recognise. |
| IDX20803 |
IDX20803: Unable to obtain configuration from: 'https://login.example.com/.well-known/openid-configuration' |
The OpenID Connect / JWT bearer middleware couldn’t download the identity provider’s metadata, so it can’t va… |
| IDX10223 |
Bearer error="invalid_token": IDX10223: Lifetime validation failed. The token is expired / IDX10501: Signature validation failed |
ASP.NET Core’s JwtBearer middleware rejected the token — expired (IDX10223), signature key not found (IDX1050… |
| ConnectionString property has not been initialized |
InvalidOperationException: The ConnectionString property has not been initialized |
The database connection was opened with an empty connection string — the configuration key wasn’t found. |
| is an invalid start of a value |
System.Text.Json: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0 |
The response wasn’t JSON — usually an HTML error page, login redirect, or an empty body was passed to the JSO… |
| Unable to create a 'DbContext' |
dotnet ef: Unable to create a 'DbContext' of type ''. The exception 'Unable to resolve service for type DbContextOptions' was thrown while attempting to create an instance |
dotnet ef migrations couldn’t construct your DbContext at design time. |
| CS0103 |
error CS0103: The name 'x' does not exist in the current context |
The identifier isn’t declared in scope — typo, variable declared in another block, missing using static, or a… |
| NETSDK1045 |
error NETSDK1045: The current .NET SDK does not support targeting .NET 8.0. Either target .NET 7.0 or lower, or use a version of the .NET SDK that supports .NET 8.0 |
The installed .NET SDK is older than the project’s TargetFramework. |
| Session has not been configured |
InvalidOperationException: Session has not been configured for this application or request |
Code used HttpContext.Session, but session services/middleware aren’t registered — or UseSession runs after t… |
| Failed to complete negotiation with the server |
SignalR: Error: Failed to complete negotiation with the server / Failed to start the connection |
The SignalR client couldn’t negotiate with the hub — wrong URL, CORS, authentication, or a proxy that blocks… |
| You must install or update .NET to run this application |
The framework 'Microsoft.NETCore.App', version '8.0.0' (x64) was not found. You must install or update .NET to run this application |
The machine doesn’t have the .NET runtime version the app was built for (framework-dependent deployment). |
| CS1061 |
error CS1061: 'X' does not contain a definition for 'Y' and no accessible extension method 'Y' accepting a first argument of type 'X' could be found |
The member you call doesn’t exist on that type — typo, wrong type, missing using for an extension method, or… |
| MSB3027 |
error MSB3027: Could not copy "obj\...\App.dll" to "bin\...\App.dll". Exceeded retry count of 10. The file is locked by: "App (1234)" |
The build can’t overwrite the output DLL/EXE because the app (or IIS Express, a test runner, antivirus) is st… |
| 500.19 |
HTTP Error 500.19 - Internal Server Error: The requested page cannot be accessed because the related configuration data for the page is invalid |
IIS can’t read web.config — usually the ASP.NET Core Hosting Bundle (AspNetCoreModuleV2) or URL Rewrite modul… |
| actively refused it |
HttpRequestException: No connection could be made because the target machine actively refused it (localhost:5001) |
Nothing is listening on the host/port the client called — the API isn’t running, uses another port, or the co… |
| JsonException object cycle |
JsonException: A possible object cycle was detected |
System.Text.Json found a reference loop (e.g. Order → Customer → Orders → …) while serialising, usually from… |
| Unexpected character encountered while parsing value |
Newtonsoft.Json.JsonReaderException: Unexpected character encountered while parsing value: <. Path '', line 0, position 0 |
Json.NET was given text that isn’t valid JSON — commonly HTML, a BOM, or an already-serialized string. |
| Failed to bind to address |
System.IO.IOException: Failed to bind to address http://127.0.0.1:5000: address already in use |
Kestrel can’t listen because another process (often a previous run of the same app) uses the port. |
| The JSON value could not be converted |
System.Text.Json.JsonException: The JSON value could not be converted to System.Int32. Path: $.id |
System.Text.Json couldn’t map a JSON value to the target .NET type — e.g. "42" (string) to int, null to a non… |
| The request matched multiple endpoints |
AmbiguousMatchException: The request matched multiple endpoints. Matches: X, Y |
Two controller actions or minimal API endpoints have the same route and HTTP method. |
| A second operation was started on this context |
EF Core: A second operation was started on this context instance before a previous operation completed |
The same DbContext was used concurrently — async calls not awaited, parallel tasks sharing a context, or a co… |
| CS0246 |
error CS0246: The type or namespace name 'X' could not be found (are you missing a using directive or an assembly reference?) |
The compiler doesn’t know the type — missing using, missing NuGet package/project reference, or a target fram… |
| Sequence contains no elements |
InvalidOperationException: Sequence contains no elements / more than one element |
LINQ `First()`, `Single()`, `Last()` or `Max()` ran on an empty sequence (or `Single()` found several items). |
| System.Drawing.Common is not supported |
PlatformNotSupportedException: System.Drawing.Common is not supported on this platform |
Since .NET 6, System.Drawing.Common only works on Windows; on Linux/macOS (including Docker) it throws. |
| Stack overflow |
.NET: Stack overflow. / Process is terminated due to StackOverflowException |
Infinite or very deep recursion exhausted the thread’s stack; .NET can’t catch it and terminates the process. |
| CORS wildcard with credentials |
ASP.NET Core: The CORS protocol does not allow specifying a wildcard (any) origin and credentials at the same time |
The CORS policy uses AllowAnyOrigin() together with AllowCredentials(), which browsers forbid. |
| The LINQ expression could not be translated |
EF Core: The LINQ expression 'x' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly |
The query uses C# code EF Core can’t convert to SQL — custom methods, unsupported string/date functions, or c… |
| No authenticationScheme was specified |
InvalidOperationException: No authenticationScheme was specified, and there was no DefaultChallengeScheme found |
[Authorize] triggered a challenge, but no authentication handler/default scheme is configured. |
| No database provider has been configured |
InvalidOperationException: No database provider has been configured for this DbContext |
EF Core created a DbContext without UseSqlServer/UseNpgsql/UseSqlite — typically because the context was newe… |
| OutOfMemoryException |
System.OutOfMemoryException: Exception of type 'System.OutOfMemoryException' was thrown |
The process couldn’t allocate memory — large in-memory data, leaks, 32-bit process limits, or container memor… |
| antiforgery token could not be decrypted |
ASP.NET Core: The antiforgery token could not be decrypted / The required antiforgery cookie is not present (400 Bad Request) |
Form POST failed antiforgery validation — the token is missing, the cookie is missing, or Data Protection key… |
| The SSL connection could not be established |
HttpRequestException: The SSL connection could not be established, see inner exception. (AuthenticationException: The remote certificate is invalid) |
HttpClient couldn’t complete the TLS handshake — untrusted/self-signed certificate, name mismatch, expired ce… |
| Collection was modified |
InvalidOperationException: Collection was modified; enumeration operation may not execute |
A list/dictionary was changed while a foreach loop was iterating it. |
| cannot be tracked because another instance with the same key |
The instance of entity type 'X' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked |
Two different objects with the same primary key were attached to one DbContext. |
| Unable to configure HTTPS endpoint |
Unable to configure HTTPS endpoint. No server certificate was specified, and the default developer certificate could not be found or is out of date |
Kestrel was told to listen on HTTPS but has no certificate — the ASP.NET Core dev certificate is missing/expi… |
| CS8618 |
warning CS8618: Non-nullable property 'X' must contain a non-null value when exiting constructor. Consider declaring the property as nullable |
With nullable reference types enabled, a non-nullable reference property isn’t initialised by the constructor. |