restsharp/RestSharp · error · ArgumentException

Parameter cannot be an empty string

Error message

Parameter cannot be an empty string

What it means

Thrown by Ensure.NotEmptyString when the supplied value, after coercion to a string, is null, empty, or whitespace. This is a shared guard used across the library (e.g. ContentType construction, parameter names) to reject blank string arguments.

Source

Thrown at src/RestSharp/Ensure.cs:24

// 
// http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

namespace RestSharp;

static class Ensure {
    public static T NotNull<T>(T? value, [InvokerParameterName] string name) => value ?? throw new ArgumentNullException(name);

    public static string NotEmptyString(object? value, [InvokerParameterName] string name) {
        var s = value as string ?? value?.ToString();
        if (s == null) throw new ArgumentNullException(name);

        return string.IsNullOrWhiteSpace(s) ? throw new ArgumentException("Parameter cannot be an empty string", name) : s;
    }
}

View on GitHub (pinned to 6a50821692)

Solutions

1. Ensure the argument passed to the guarded API is a non-empty, non-whitespace string.
2. Validate inputs at the call site before invoking the RestSharp API.
3. Supply sensible defaults when config-driven strings may be empty.

Example fix

// before
var ct = (ContentType)(config.ContentType ?? ""); // empty config

// after
var ctStr = string.IsNullOrWhiteSpace(config.ContentType) ? "application/json" : config.ContentType;
var ct = (ContentType)ctStr;

if (string.IsNullOrWhiteSpace(value)) throw new ArgumentException("Value must be a non-empty, non-whitespace string", nameof(value));

static bool IsNonEmptyString(string? s) => !string.IsNullOrWhiteSpace(s);

try { /* RestSharp API call guarded by Ensure.NotEmptyString */ } catch (ArgumentException ex) when (ex.Message.Contains("empty string")) { /* supply a valid default */ }

When it happens

Trigger: Calling an API that delegates to Ensure.NotEmptyString with a null, empty, or whitespace-only string argument, such as constructing a ContentType from a blank string, or passing a blank parameter name.

Common situations: Reading content-type or parameter name from config that resolved to empty; passing string.Empty inadvertently; whitespace-only values from trimmed user input.

Related errors

• value must be non-negative
• Duplicate header names exist: {duplicateKeys}
• Invalid content type string
• Request body serialized to null
• AdvancedResponseWriter is not null. Only one response writer

AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/73d67cc12d595cfb. Report an issue: GitHub.